Fabrication of p-channel field-effect transistor for reducing junction capacitance

ABSTRACT

An IGFET ( 40  or  42 ) has a channel zone ( 64  or  84 ) situated in body material ( 50 ). Short-channel threshold voltage roll-off and punchthrough are alleviated by arranging for the net dopant concentration in the channel zone to longitudinally reach a local surface minimum at a location between the IGFET&#39;s source/drain zones ( 60  and  62  or  80  and  82 ) and by arranging for the net dopant concentration in the body material to reach a local subsurface maximum more than 0.1 μm deep into the body material but not more than 0.1 μm deep into the body material. The source/drain zones ( 140  and  142  or  160  and  162 ) of a p-channel IGFET ( 120  or  122 ) are provided with graded-junction characteristics to reduce junction capacitance, thereby increasing switching speed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 09/947,012,filed Sep. 4, 2001, now U.S. Pat. No. 6,599,804, which is a division ofU.S. patent application Ser. No. 09/540,442, filed Mar. 31, 2000, nowU.S. Pat. No. 6,548,842.

FIELD OF USE

This invention relates to semiconductor technology and, in particular,to field-effect transistors (“FETs”) of the insulated-gate type. All ofthe insulated-gate FETs (“IGFETs”) described below are surface-channelenhancement-mode IGFETs except as otherwise indicated.

BACKGROUND

An IGFET is a semiconductor device in which a gate dielectric layerelectrically insulates a gate electrode from a channel zone that extendsbetween a source and a drain in a semiconductor body. The channel zonein an-enhancement-mode IGFET is part of a body region, commonly termedthe substrate or substrate region, that forms respective pn junctionswith the source and drain. In an enhancement-mode IGFET, the channelzone consists of all the semiconductor body material situated betweenthe source and drain. During operation of an enhancement-mode IGFET,charge carriers move from the source to the drain through a channelinduced in the channel zone along the upper semiconductor surface. Thechannel length is the distance between the source and drain along theupper semiconductor surface.

Over the last forty years, the minimum value of IGFET channel length hasdecreased generally in the manner prescribed by Moore, “Progress inDigital Integrated Electronics,” Tech. Dig., 1975 Int'l Elec. Devs.Meeting, 1-3 Dec. 1975, pages 11-13. Per Moore's “law”, the minimumchannel length decreases roughly in proportion to a factor of 1/{squareroot over (2)} (approximately 0.7) every three years. IGFETs employed instate-of-the-art integrated circuits (“ICs”) manufactured atvolume-production quantities today have minimum channel lengthsconsiderably less than 1 μm, typically 0.25 μm and moving towards 0.18μm. The minimum channel length for volume-production ICs is expected tobe roughly 0.1 μm in eight to ten years.

An IGFET that behaves generally in the way prescribed by the classicalmodel for an IGFET is often characterized as a “long-channel” device. AnIGFET is described as a “short-channel” device when the channel lengthis shortened to such an extent that the IGFET's behavior deviatessignificantly from the behavior of the classical IGFET model. Bothshort-channel and long-channel IGFETs are variously employed in ICs.Because drive current generally increases with decreasing channellength, the great majority of IGFETs used in very large scaleintegration applications are laid out to have as small a channel lengthas can be reliably produced with the available lithographic technology.

One short-channel effect is roll-off of the threshold voltage. See (a)Yau, “A Simple Theory to Predict the Threshold Voltage of Short-ChannelIGFET's”, Solid-State Electronics, October 1974, pages 1059-1069, and(b) Liu et al, “Threshold Voltage Model for Deep-SubmicrometerMOSFET's”, IEEE Trans. Elec. Devs., Vol. 40, No. 1, January 1993, pages86-95. The threshold voltage is the value of gate-to-source voltage atwhich an IGFET switches between its on and off states for givendefinitions of the on and off states.

FIG. 1 illustrates a typical example of how threshold voltage VT rollsoff for a conventional n-channel enhancement-mode IGFET whoseparameters, other than channel length L, are fixed. As FIG. 1 indicates,threshold voltage V_(T) has relatively little variation in thelong-channel regime where channel length L is greater than transitionvalue L_(X) approximately equal to 0.4 μm here. When channel length Ldrops below L_(X), the IGFET enters the short-channel regime in whichthreshold voltage V_(T) rolls off sharply to zero.

In designing IGFETs with increasingly reduced channel length, animportant trade-off is between drive current and leakage current. Thedrive current, preferably high, is the current that flows between thesource and drain when the IGFET is turned fully on. The leakage current,preferably low, is the current that flows between the source and drainwhen the IGFET is turned off with the gate electrode electricallyshorted to the source. Decreasing the channel length typically leads toan increase in the drive current. However, the leakage current alsotypically increases when the channel length is reduced.

Due to the foregoing trade-off, a short-channel IGFET is typicallydesigned so that channel length L is of a value close to where thresholdvoltage V_(T) starts to roil off sharply to zero. An L value of 0.25 μmsatisfies this requirement in FIG. 1. The resulting V_(T) value ofslightly more than 0.5 V is sufficiently high to enable a 0.25-μmn-channel IGFET to switch reliably between its on and off states.However, threshold voltage V_(T) for an n-channel IGFET having an Lvalue of 0.10 μm, as occurs in the next generation of IGFETs, isapproximately 0.2 V. This is too low to be able to reliably turn such a0.18-μm IGFET off at zero gate-to-source voltage, especially in light oftypical manufacturing variations.

The scaling principles developed by Dennard et al, “Design ofIon-Implanted MOSFET's with Very Small Physical Dimensions” IEEE J.Solid-State Circs., Vol. SC-9, No. 5, October 1974, pages 256-268, havebeen utilized in downsizing IGFETS. In brief, Dennard et al specifiesthat IGFET dimensions are to be reduced approximately in proportion to agiven scaling factor as the average net dopant concentration in thechannel zone, i.e., the semiconductor body material situated between thesource and drain in an enhancement-mode IGFET, is increased by thescaling factor. The voltages across various parts of thereduced-dimension IGFET are also generally to be reduced in proportionto the scaling factor.

The scaling theory of Dennard et al functions relatively well down tochannel length in the vicinity of 1 μm. Unfortunately, certain scalinglimitations are encountered when the channel length is reducedsignificantly below 1 μm. For example, electron tunneling effectspreclude reducing the gate dielectric thickness to the value prescribedby the scaling theory.

Also, when the threshold voltage is to be adjusted by simply implantingthe channel zone with ions of the same conductivity type as the channelzone, it is typically preferable that the threshold adjust implant bedistinguishable from the vertical dopant profile in the bulk of thechannel zone. In scaling an IGFET to channel length significantly lessthan 1 μm according to the theory of Dennard et al, the threshold adjustimplant merges inseparably into the vertical dopant profile in the bulkof the channel zone, thereby simply raising the average net dopantconcentration in the channel zone by an approximately fixed amount thatis largely independent of channel length. Attempting to extend thescaling theory to channel length significantly less than 1 μm does notwork well.

Various techniques have been utilized to improve the performance ofIGFETs, including those operating in the short-channel regime, as IGFETdimensions are reduced. One performance-improvement technique involvesproviding an IGFET with a two-part drain for reducing hot carrierinjection. FIG. 2 illustrates such a conventional n-channelenhancement-mode IGFET 10 created from a monocrystalline siliconsemiconductor body having region 12 of lightly doped p-type bodymaterial. IGFET 10 has n-type source 14, n-type drain 16, interveningp-type channel zone 18, gate electrode 20, gate dielectric layer 22, andgate sidewall spacers 24 and 26. Drain 16 consists of heavily doped mainportion 16M and more lightly doped extension 16E. Source 14 similarlyconsists of heavily doped main portion 14M and more lightly dopedextension 14E. When IGFET 10 is turned on, electrons travel from source14 to drain 16 by way of a thin channel induced in channel zone 18 alongthe upper semiconductor surface.

A pair of depletion regions extend respectively along the drain/body andsource/body junctions. Under certain conditions, especially when thechannel length is small, the drain depletion region can extend laterallyto the source depletion region and merge with it below the uppersemiconductor surface. This phenomenon is termed punchthrough. If thedrain depletion region punches through to the source depletion region,the operation of the IGFET cannot be controlled with the gate electrode.Accordingly, punchthrough normally needs to be avoided.

One conventional technique for inhibiting punchthrough as channel lengthis reduced, and also for shifting threshold voltage roll-off to shorterchannel length, is to increase the dopant concentration of the channelzone in a pocket portion along the source. See Ogura et al, “A HalfMicron MOSFET Using Double Implanted LDD,” IEDM Tech. Dig., Int'l Elec.Devs. Meeting, 11-15 Dec. 1982, pages 718-721. As an artifact ofcreating the increased-concentration pocket portion along the source,the dopant concentration in the channel zone is commonly increased in acorresponding pocket portion along the drain. Per Codella et al, “HaloDoping Effects in Submicron DI-LDD Device Design,” IEDM Tech. Dig.,Int'l Elec. Devs. Meeting, 1-4 Dec. 1985, pages 230-233, the pocketportions are commonly referred to as “halo”.

Increasing the dopant concentration in a halo pocket along the sourcereduces the thickness of the channel-zone part of the source depletionregion, thereby deferring the onset of punchthrough. As the channellength is reduced, the halo pockets along the source and drain getcloser together so as to increase the average net dopant concentrationin the channel zone. This causes the threshold voltage to increase,thereby partially counteracting threshold voltage roll-off at shortchannel length.

FIG. 3 depicts a conventional n-channel enhancement-mode halo IGFET 30as configured in Ogura et al or Codella et al. Except as describedbelow, IGFET 30 contains the same regions as IGFET 10 in FIG. 2. Asshown in FIG. 3, channel zone 18 in n-channel IGFET 30 includes a pairof p-type halo pocket portions 31 and 32 doped more heavily than theremainder of channel zone 18. Halo pockets 31 and 32 are situated alongthe inner boundaries of source 14 and drain 16 so as to inhibitpunchthrough. Metal silicide layers 33, 34, and 35 respectively contactmain source portion 14M, main drain portion 16M, and gate electrode 20.

Halo pockets 31 and 32 can be created in various ways. For example,p-type halo dopant is typically ion implanted through the uppersemiconductor surface into the semiconductor body using gate electrode20 as an implantation shield. The halo implant can be performed roughlyperpendicular to the upper semiconductor surface as indicated in Oguraet al.

The halo implant can also be performed at a substantial angle to aperpendicular to the upper semiconductor surface. In this regard, see(a) Su, “Tilt Angle Effect on Optimizing HALO PMOS Performance,” 1997Int'l Conf. Simulation Semicon. Procs. and Devs., 8-10 Sep. 1997, pages33-36, (b) Rodder et al, “A Sub-0.18 μm Gate Length CMOS Technology forHigh Performance (1.5 V) and Low Power (1.0 V),” IEDM Tech. Dig., Int'lElec. Devs. Meeting, 8-11 Dec. 1996, pages 563-566, (c) Hori, “A 0.1-μmCMOS Technology with Tilt-Implanted Punchthrough Stopper (TIPS),” IEDMTech. Dig., Int'l Elec. Devs. Meeting, 11-14 Dec. 1994, pages 75-78, and(d) Hwang et al, “Degradation of MOSFETs Drive Current Due to Halo IonImplantation,” IEDM Tech. Dig., Int'l Elec. Devs. Meeting, 8-11 Dec.1996, pages 567-570.

The threshold voltage of n-channel IGFET 30 is adjusted by introducingp-type dopant, typically boron, into a portion 36 of channel zone 18.The threshold adjust dopant typically has a retrograde dopantconcentration profile in that the maximum concentration of the thresholdadjust dopant occurs below the upper semiconductor surface. The maximumconcentration of the threshold adjust dopant may occur in, or below, adepletion region that extends across channel zone 18 along the uppersemiconductor surface during IGFET operation. For example, theinformation presented in Ogura et al indicates that the maximumconcentration of the threshold adjust dopant in Ogura et al occurs inthe surface depletion region slightly more than 0.1 μm below the uppersemiconductor surface. As a result, the magnitude of the thresholdvoltage adjustment in Ogura et al is determined primarily by the dosageof the p-type dopant.

Shahidi et al, “High Performance Devices for a 0.15 μm CMOS Technology,”IEEE Elect. Dev. Lett., Vol. 14, No. 10, October 1993, pages 466-468,and Taur et al, “High Performance 0.1 μm CMOS Devices with 1.5 V PowerSupply,” IEDM Tech. Dig., Int'l Elec. Devs. Meeting 1993, pages 127-130,describe embodiments of IGFET 30 in which p-type dopant is introducedinto channel zone 18 in such a manner that the maximum concentration ofthe implanted dopant occurs 0.07-0.10 μm below the upper semiconductorsurface. Instead of boron, Shahidi et al employs indium, aslow-diffusing species, as the p-type dopant for the implant intochannel zone 18. Shahidi also employs indium for the halo pockets. Whileindium may provide profile steepness and better short-channel behavior,indium is not commonly used in semiconductor manufacturing processes andmay cause process implementation difficulties.

Hwang et al, cited above, describes an n-channel IGFET in which a p-typeimplant is performed relatively deep into the channel zone in order toalleviate punchthrough. The depth of the maximum concentration of thep-type anti-punchthrough implant in this IGFET of Hwang et al appears tooccur below the channel surface depletion region. In comparing a haloIGFET to an IGFET having an anti-punchthrough implant but no halopocket(s), Hwang et al determines that less threshold voltage roll-offoccurs in the halo IGFET.

Conventional semiconductor manufacturing processes achieve varyingdegrees of success in avoiding punchthrough and alleviating thresholdvoltage roll-off. It is desirable to have an IGFET structure andfabrication technique for overcoming these problems. In so doing, it isdesirable to avoid use of semiconductor dopants which are not widelyemployed in volume-production IC fabrication and which could causemanufacturing difficulties. It is also desirable to have a semiconductorstructure in which different IGFETs can readily be provided withdifferent threshold voltages.

GENERAL DISCLOSURE OF THE INVENTION

The present invention furnishes an insulated-gate field-effecttransistor whose doping is controlled to alleviate threshold voltageroll-off and avoid punchthrough at short channel length. The averagedoping in the channel zone of the present IGFET changes with channellength in such a manner that the variation of threshold voltage withchannel length in the short-channel operational regime where thresholdvoltage roll-off occurs in an otherwise conventional IGFET isconsiderably less than in the conventional IGFET. The channel length ofthe present IGFET can thereby be reduced below the normal value at whichthreshold voltage roll-off occurs without having the magnitude of thethreshold voltage drop significantly. Fabrication variations that resultin unintended channel length differences do not lead to substantialthreshold voltage differences in the present IGFET.

More particularly, the channel zone of the present IGFET is situated inbody material of a semiconductor body. The channel zone laterallyseparates a pair of source/drain zones situated in the semiconductorbody along its upper surface. The source/drain zones form pn junctionswith the body material. A gate electrode overlies a gate dielectriclayer above the channel zone.

The dopant profile of the present IGFET has two importantcharacteristics, one directed primarily towards alleviatingshort-channel threshold voltage roll-off and the other directedprimarily towards alleviating punchthrough. The first characteristic isthat the net dopant concentration of the channel zone along the uppersemiconductor surface longitudinally reaches a local surface minimumbetween the source/drain zones. The channel dopant profile along theupper semiconductor surface in the longitudinal direction, i.e., alongthe IGFET's channel length, thus typically has at least a half-saddleshape, typically a full saddle shape.

The average value of the channel zone's net dopant concentrationnormally increases with decreasing channel length for a given amount ofsemiconductor dopant (per unit channel width) producing the channelsurface dopant profile. By arranging for the average net dopantconcentration of the channel zone to vary in this manner, the magnitudeof the threshold voltage of the present IGFET increases slowly withdecreasing channel length in passing through the value of channel lengthat which short-channel threshold voltage roll-off starts to occur in anotherwise similar conventional IGFET. Since the magnitude of thethreshold voltage of the present IGFET increases with decreasing channellength in this short-channel operational regime rather than decreasingsharply with decreasing channel length, the variation of the thresholdvoltage with channel length in the short-channel regime is considerablyless than in the conventional IGFET. The onset of threshold voltageroll-off in the present IGFET is thereby advantageously shifted to lowerchannel length.

The second important characteristic of the dopant profile of the presentIGFET is that the net dopant concentration of the body material reachesa local subsurface maximum more than 0.1 μm below the uppersemiconductor surface but not more than 0.4 μm below the uppersemiconductor surface. Also, the local subsurface maximum of the bodymaterial's net dopant concentration occurs below a channel surfacedepletion region that extends along the upper semiconductor surface intothe channel zone. The local subsurface maximum of the body material'snet dopant concentration normally exceeds the local surface minimum ofthe channel zone's net dopant concentration. The vertical dopant profilethrough the local subsurface minimum is therefore of a retrogradenature.

The semiconductor dopant which produces the local subsurface maximum inthe body material's net dopant concentration causes the thicknesses ofthe body-material portions of the depletion regions along thesource/drain zones to be reduced at a given voltage between thesource/drain zones. The source/drain zone acting as the drain (at anyparticular time) is thereby inhibited from punching through to thesource/drain zone acting as the source.

A feature of the present invention is that the gate dielectric layer istypically 2-10 nm in thickness. While the gate dielectric layer normallycontains atoms of semiconductor material, typically silicon, and atomsof oxygen combined to form semiconductor oxide, typically silicon oxide,atoms of nitrogen may also be incorporated into the gate dielectriclayer. That is, the gate dielectric layer may contain semiconductoroxynitride, typically silicon oxynitride.

In a structure containing two implementations of the present IGFET, thechannel zones of the two IGFETs can readily be of sufficiently differentlength that the IGFETs differ significantly in threshold voltage. Withthe local surface minima in the net dopant concentrations of the channelzones being produced by approximately equal amounts of semiconductordopant (per unit channel width) as occurs when IGFETs are manufacturedunder largely identical fabrication process conditions, the shorterchannel zone normally has the higher average net dopant concentration.Accordingly, the FET with the shorter channel zone normally has thehigher magnitude of threshold voltage.

While two IGFETs with significantly different threshold voltages can beprovided in the same structure of the invention by suitably choosing thechannel lengths, two such IGFETs can also be provided in differentstructures. In other words, identical fabrication process conditions canbe employed to provide separate structures with IGFETs of significantlydifferent threshold voltage by simply making the channel lengthssufficiently different.

Fabrication of an IGFET in accordance with the invention begins with asemiconductor body having body material of a first conductivity type.First semiconductor dopant, referred to here as the anti-punchthrough(“APT”) dopant, of the first conductivity type is introduced into thebody material. This doping step is performed in such a manner that,subsequent to providing an electrically insulated gate electrode above aportion of the body material intended to be a channel zone, the maximumconcentration of the APT dopant occurs more than 0.1 μm into the bodymaterial but not more than 0.4 μm into the body material. The maximumconcentration of the APT dopant also occurs below the location where achannel surface depletion region extends into the channel zone duringIGFET operation. The doping characteristic achieved with the APT dopantprovides punchthrough protection.

The APT dopant is normally ion implanted into the body material. Sincethe maximum concentration of the APT dopant occurs below the channelsurface depletion region, substantially only part of the “head” (upperportion) of the implant of the APT dopant affects the IGFET's thresholdvoltage. Consequently, the threshold voltage can be controlled largelyindependent of channel length by simply adjusting the implantationenergy. As the implantation energy increases, less of the APT dopantaccumulates in the channel surface depletion region, thereby causing themagnitude of the threshold voltage to be reduced. The ability to controlthe threshold voltage by adjusting the implantation energy providesenhanced flexibility over a conventional threshold adjust implant inwhich adjustment of the implantation dosage is the primary mechanism forcontrolling the threshold voltage.

Later in the fabrication process after forming the gate electrode,second semiconductor dopant, referred to here as the halo dopant, of thefirst conductivity type is introduced into at least the intendedchannel-zone portion of the body material. Also, dopant, referred tohere as the source/drain dopant, of a second conductivity type oppositeto the first conductivity type is introduced into the semiconductor bodyto form a pair of source/drain zones laterally separated by the channelzone. These doping operations are performed in a way that enables theconcentration of the halo dopant to longitudinally reach a local surfaceminimum in the channel zone along the upper semiconductor surface. Thedoping characteristic provided by the halo dopant alleviatesshort-channel threshold voltage roll-off and assists the APT dopant inproviding punchthrough protection.

The halo and source/drain dopants enter the body material by passingthrough the upper semiconductor surface. A shield formed with at leastthe gate electrode is preferably utilized to largely prevent the haloand source/drain dopants from passing through the section of the uppersemiconductor surface underlying at least the gate electrode. The dopingoperation with the source/drain dopant may be performed in at least twoseparate steps so as to form each source/drain zone as a main portionand a more lightly doped extension. The channel zone is terminated bythe source/drain extensions along the upper semiconductor surface.

The halo dopant forms one or more pocket portions akin to halo when thehalo dopant is introduced into the intended channel-zone portion of thebody material using a dopant-blocking shield consisting of at least thegate electrode. Angled ion implantation is preferably employed tointroduce the halo dopant into the channel-zone portion of the bodymaterial. Specifically, ions of the halo dopant are implanted into thechannel-zone portion of the body material at an average tilt angle of atleast 15° relative to a direction generally perpendicular to the uppersemiconductor surface. By using angled ion implantation, the halo dopantcan be placed where it yields a large reduction in short-channelthreshold voltage roll-off.

An additional feature of the present invention is that each source/drainzone can have a vertically “graded” junction characteristic. That is, inmoving upward from the pn junction between a source/drain zone and theadjoining body material, the net dopant concentration in a verticalcross-section through the source/drain zone rises less sharply, on theaverage, to the maximum value of the net dopant concentration in thatvertical cross-section than what would occur if the semiconductor dopantwhich defines the source/drain zone across that vertical cross-sectionwere ion implanted at largely a single energy. The graded-junctioncharacteristic reduces the junction capacitance, thereby leading to anincrease in the IGFET switching speed.

The graded-junction characteristic is attained by configuring eachsource/drain zone to have a main source/drain portion and a more lightlydoped lower source/drain portion which underlies, and is verticallycontinuous with, the main source/drain portion. In fabricating thepresent IGFET to have a vertically graded pn junction, firstsemiconductor of the second conductivity type is introduced into thesemiconductor body at a first dosage and to a first average depth (belowthe upper semiconductor surface). Second semiconductor dopant of thesecond conductivity type is introduced into the semiconductor body at asecond dosage less than the first dosage and to a second average depthgreater than the first average depth such that the first dopant of thesecond conductivity type defines the main source/drain portions whilethe second dopant of the second conductivity type defines the morelightly doped lower source/drain portions.

More generally, a p-channel IGFET configured in accordance with theinvention contains a channel zone situated in a semiconductor bodyhaving an upper surface. A gate electrode overlies a gate dielectriclayer above the channel zone. The channel zone laterally separates apair of p-type source/drain zones of the semiconductor body. Each p-typesource/drain zone has a main source/drain portion and a more lightlydoped lower source/drain portion underlying, and vertically continuouswith, the main portion. Consequently, the p-channel IGFET has agraded-junction characteristic.

Fabrication of such a graded junction p-channel IGFET entails providingthe gate electrode over the gate dielectric layer above a location forthe channel zone in the semiconductor body. First, p-type semiconductordopant is introduced, typically by ion implanting a species of the firstp-type dopant, into the semiconductor body at a first dosage and to afirst average depth. Second, p-type semiconductor dopant is introduced,likewise typically by ion implanting a species of the second p-typedopant, into the semiconductor body at a second dosage less than thefirst dosage and to a second average depth greater than the firstaverage depth. The dopant-introduction steps are controlled so that thefirst p-type dopant defines the main portions of the source/drain zones,so that the second p-type dopant defines the lower source/drainportions, and so that the lower portions are more lightly doped than,and respectively vertically continuous with, the main portions.

The p-type dopant species, typically a boron-containing compound such asboron difluoride, employed in defining the main portions of thesource/drain zones is preferably of greater molecular weight than thep-type dopant species, typically elemental boron, utilized in definingthe lower source/drain portions. With boron being an atom of relativelylow molecular weight and with the main source/drain portions beingshallower than the lower source/drain portions, one reason for thisdifference in dopant species is that it is difficult to reliably controltypical commercially available state-of-the-art ion-implantationequipment at the low implantation energy (e.g., 5 KeV) which wouldgenerally be appropriate if elemental boron were the p-type speciesutilized to define the main source/drain portions. Performing theimplantation to define the main source/drain portions with a p-typedopant species, e.g., boron difluoride, of greater molecular weight thanthe p-type dopant species, e.g., elemental boron, utilized to define thelower source/drain portions thus facilitates accurately forming thegraded-junction characteristic.

In short, an IGFET configured and fabricated according to the inventionnormally has a dopant profile that causes the onset of short-channelthreshold voltage roll-off to be shifted to lower channel length thanoccurs in an otherwise similar conventional IGFET. Punchthrough issubstantially alleviated in the present invention. The threshold voltagecan be controlled largely independent of channel length by adjusting theenergy of implanted ions of dopant utilized to avoid punchthrough. Forthe same, or substantially the same, fabrication process conditions,IGFETs of significantly different threshold voltage are achieved bysimply utilizing appropriately different channel lengths. An IGFET,especially a p-type channel device, configured and fabricated accordingto the invention, can be provided with a graded-junction characteristic.The invention thus provides a large advance over the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of threshold voltage as a function of channel lengthfor a conventional n-channel IGFET.

FIGS. 2 and 3 are front cross-sectional views of conventional n-channelIGFETs.

FIG. 4 is a graph of long-channel threshold voltage as a function ofchannel dopant concentration for an n-channel IGFET.

FIG. 5 is a graph of average channel dopant concentration as a functionof channel length for IGFETs manufactured according to the invention.

FIG. 6 is a graph of threshold voltage as a function of channel lengthfor n-channel IGFETs manufactured according to the invention and forn-channel IGFETs manufactured at various fixed values of average netchannel dopant concentration.

FIG. 7 is a graph of channel dopant concentration as a function ofchannel length for IGFETs manufactured according to the invention atvarious degrees of short-channel compensation.

FIG. 8 is a graph of threshold voltage as a function of channel lengthfor n-channel IGFETs manufactured according to the invention at variousdegrees of short-channel compensation and for n-channel IGFETsmanufactured at various fixed values of average net channel dopantconcentration.

FIGS. 9a and 9 b are front cross-sectional views of respectivesymmetrical long and short n-channel IGFETs configured and manufacturedaccording to the invention.

FIGS. 10a and 10 b are exemplary graphs of net dopant concentrationalong the upper semiconductor surface as a function of longitudinalhorizontal distance from the channel center for the respective long andshort n-channel IGFETs of FIGS. 9a and 9 b.

FIG. 11 is an exemplary graph of APT dopant concentration as a functionof vertical distance into the body material at the channel center forthe n-channel IGFETs of FIGS. 9a and 9 b.

FIGS. 12a and 12 b are exemplary graphs of net dopant concentration as afunction of vertical distance into the body material at the channelcenter for the respective long and short n-channel IGFETs of FIGS. 9aand 9 b.

FIGS. 13a and 13 b are front cross-sectional views of additionalrespective symmetrical long and short n-channel IGFETs configured andmanufactured according to the invention.

FIGS. 14a and 14 b are exemplary graphs of net dopant concentration as afunction of vertical distance into the body material at the channelcenter for the respective long and short n-channel IGFETs of FIGS. 13aand 13 b.

FIGS. 15a and 15 b are front cross-sectional views of respectivesymmetrical long and short p-channel IGFETs configured and manufacturedaccording to the invention, typically along with the n-channel IGFETs ofFIGS. 9a and 9 b.

FIGS. 16a-16 m are front cross-sectional views representing steps in afabrication process according to the invention for manufacturing theIGFETs of FIGS. 9a, 9 b, 15 a, and 15 b.

FIG. 17 is an exemplary graph of temperature as a function of time forgrowing a gate dielectric layer in the fabrication process of FIGS.16a-16 m.

FIGS. 18a and 18 b are front cross-sectional views of respectivesymmetrical long and short p-channel IGFETs configured and manufacturedaccording to the invention.

FIG. 18c is an exemplary graph of net dopant concentration as a functionof vertical distance into a source/drain zone along a vertical crosssection through the center of the main portion of the source/drain zonein the p-channel IGFET of FIGS. 18a or 18 b.

FIG. 19a is an experimental graph of threshold voltage as a function ofdrawn channel length for symmetrical n-channel IGFETs manufacturedaccording to the invention at various APT and halo dosages.

FIG. 19b is an experimental graph of extrapolated leakage currentdensity as a function of drive current density for symmetrical n-channelIGFETs manufactured according to the invention at the APT and halodosages of FIG. 19a.

FIG. 20 is an experimental graph of threshold voltage as a function ofdrawn channel length for symmetrical n-channel IGFETs manufacturedaccording to the invention and for symmetrical n-channel IGFETsmanufactured with a very low halo dosage.

FIG. 21 is an experimental graph of threshold voltage as a function ofdrawn channel length for symmetrical n-channel IGFETs manufacturedaccording to the invention with silicon oxide and silicon oxynitridegate dielectric layers.

FIG. 22 is an experimental graph of threshold voltage as a function ofdrawn channel length for symmetrical n-channel IGFETs manufacturedaccording to the invention with silicon oxynitride gate dielectriclayers created with, and without, extended-time furnace anneal.

FIG. 23 is an experimental graph of threshold voltage as a function ofdrawn channel length for symmetrical n-channel IGFETs manufacturedaccording to the invention at various APT implantation energies.

FIGS. 24a and 24 b are exemplary graphs of net dopant concentration as afunction of vertical distance into the body material along verticalplanes respectively extending through the channel center and situatednear a source/drain zone for computer simulations of symmetrical longn-channel IGFETs manufactured according to the invention at two of theAPT implantation energies of FIG. 23.

FIGS. 25 and 26 are experimental graphs of absolute threshold voltage asa function of drawn channel length for respective symmetrical n-channeland p-channel IGFETs manufactured according to the invention at varioushalo implantation angles.

FIGS. 27a and 27 b are exemplary graphs of net dopant concentration as afunction of vertical distance into the body material along verticalplanes respectively extending through the channel center and situatednear a source/drain zone for computer simulations of symmetrical longn-channel IGFETs manufactured according to the invention with normalwell and high well implants.

FIG. 28 is an experimental graph of threshold voltage as a function ofdrawn channel length for symmetrical n-channel IGFETs manufacturedaccording to the invention with normal well and high implants.

FIG. 29 is a experimental graph of threshold voltage as a function ofdrawn channel length for symmetrical n-channel and p-channel IGFETsmanufactured according to the invention at two gate dielectricthicknesses.

FIG. 30 is an experimental graph of extrapolated leakage current densityas a function of drive current density for symmetrical n-channel IGFETsmanufactured according to the invention at the two gate dielectricthicknesses of FIG. 29.

FIG. 31a is an experimental graph of threshold voltage as a function ofdrawn channel length for symmetrical n-channel IGFETs manufacturedaccording to the invention at two different times of source/drain-halorapid thermal anneal.

FIG. 31b is an experimental graph of extrapolated leakage currentdensity as a function of drive current density for symmetrical n-channelIGFETs manufactured according to the invention at the two rapid thermalanneal times of FIG. 31a.

FIG. 32 is an experimental graph of areal junction capacitance as afunction of boron difluoride ion implantation energy for symmetricalp-channel IGFETs manufactured according to the invention with andwithout graded-junction characteristics.

FIG. 33 is an experimental graph of threshold voltage as a function ofdrawn channel length for symmetrical graded-junction n-channel andp-channel IGFETs manufactured according to the invention at twodifferent values of sidewall spacer thickness and, for p-channel IGFETS,at two different values of boron difluoride ion implantation energy.

FIG. 34 is a front cross-sectional view for a model of how shadowing bya gate electrode structure affects angled halo ion implantation.

FIGS. 35 and 36 are front cross-sectional and layout views for a modelof how photoresist mask shadowing affects angled halo ion implantation.The cross section of FIG. 35 is taken through plane 35—35 in FIG. 36.

FIG. 37 is a front cross-sectional view for a model of how dopantredistribution varies with channel length in three symmetrical longn-channel IGFETs configured according to the invention to be ofdifferent channel lengths.

FIG. 38 is a graph of net dopant concentration as a function oflongitudinal distance into the channel zone for each of the n-channelIGFETs in the model of FIG. 37. The dopant profile of FIG. 38 is takenalong plane 38—38 in FIG. 37.

FIG. 39 is a front cross-sectional view of the long n-channel IGFET ofFIG. 9a in which pair-enhanced diffusion of boron occurs.

FIGS. 40a and 40 b are exemplary graphs of net dopant concentration as afunction of vertical distance into the body material along verticalplanes respectively situated near a source/drain zone and extendingthrough the channel center for computer simulations of a symmetricalshort n-channel IGFET manufactured according to the invention.

FIGS. 41a and 41 b are front cross-sectional views of respectiveasymmetrical long and short n-channel IGFETs configured and manufacturedaccording to the invention.

FIGS. 42a and 42 b are exemplary graphs of net dopant concentration as afunction of longitudinal horizontal distance from the channel center forthe respective long and short n-channel IGFETs of FIGS. 41a and 41 b.

Like reference symbols are employed in the drawings and in thedescription of the preferred embodiments to represent the same, or verysimilar, item or items.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference Notation and Other Convention

The reference symbols employed below and in the drawings have thefollowing meanings:

A ≡ halo dopant atom AI ≡ pair of dopant atom A and interstitial atom Ia ≡ experimentally determined coefficient b ≡ experimentally determinedcoefficient C_((A)) ^(TOT) ≡ total concentration (single and paired) ofdopant atom A in crystal C_(GD) ≡ gate dielectric capacitance per unitarea C_(J) ≡ capacitance per unit area of pn junction between bodymaterial and source/drain zone c ≡ lateral distance from gate electrodestructure across location of source/drain zone to field-insulationregion in direction along channel lengths D_((A0)) ≡ equilibriumdiffusion constant of halo dopant atoms A DOSE_(APT) ≡ dosage of ions ofanti-punchthrough dopant DOSE_(Halo) ≡ dosage of ions of halo dopantd_(G) ≡ lateral distance between gate electrode structures in directionalong channel length of IGFET receiving halo ion implantation d_(Gmin) ≡minimum tolerable value of distance d_(G) d_(M) ≡ lateral distance fromgate electrode structure across location of source/drain zone tophotoresist mask d_(Mmax) ≡ maximum tolerable value of distance d_(M)d_(Mmin) ≡ minimum tolerable value of distance d_(M) E_(BF2) ≡implantation energy of ions of boron difluoride during formation of mainportions of p-type source/drain zones E_(G) ≡ band-gap energy betweenconduction and valence bands h_(G) ≡ height of gate electrode structureabove gate dielectric layer h_(M) ≡ height of photoresist mask abovegate dielectric layer I ≡ interstitial atom I_(DSAT) ≡ widthwise drivecurrent density or drive current per unit drain width I_(DSSX) ≡extrapolated widthwise leakage current density or extrapolated leakagecurrent per unit drain width I_(DSS0) ≡ actual widthwise leakage currentdensity or actual leakage current per unit drain width i ≡ width offield-insulation region in direction of channel lengths {overscore(J)}_((AI)) ≡ diffusion flux (current density) of interstitial- dopantpairs in crystal K ≡ spatially dependent diffusion parameter k ≡Boltzmann's constant L ≡ channel length along upper semiconductorsurface L_(C) ≡ operating value of channel length L in short-channelregime L_(DR) ≡ drawn value of channel length L as given by drawn valueof gate length L_(DRC) ≡ desired operating value of drawn channel lengthL_(DR) in short-channel regime L_(X) ≡ transition value of channellength L l ≡ normalization length N_(APT) ≡ concentration ofanti-punchthrough dopant N_(B) ≡ net dopant concentration of bodymaterial N_(C) ≡ average net dopant concentration in channel zone N_(N)≡ general net dopant concentration N_(POLY) ≡ average net dopantconcentration in gate electrode consisting of polycrystallinesemiconductor material N_(S/D) ≡ average net dopant concentration insource/drain zones n_(i) ≡ intrinsic carrier concentration Q_(ss) ≡charge per unit area at interface between gate dielectric layer andchannel zone q ≡ electronic charge R_(Ext) ≡ vertical range (averagedepth) of ions of drain-extension dopant R_(Halo) ≡ vertical range(average depth) of ions of halo dopant T ≡ temperature t_(d(INV)) ≡thickness of depletion region along surface of channel zone at inversiont_(GD) ≡ thickness of gate dielectric layer t_(Halo) ≡ lateral thicknessof halo pocket portion directly after halo implantation at depth equalto vertical range R_(Halo) t_(SP) ≡ bottom thickness of spacer alongsidewall of gate electrode V_(BI) ≡ built-in voltage across pn junctionV_(DS) ≡ drain-to-source voltage V_(DS(TH)) ≡ drain-to-source voltage atthreshold point V_(FB) ≡ flat-band voltage V_(GS) ≡ gate-to-sourcevoltage V_(T) ≡ general threshold voltage V_(T0) ≡ long-channelthreshold voltage x ≡ lateral distance from channel center along uppersemi- conductor surface toward source/drain zone in direction of channellength y ≡ depth into semiconductor body measured from upper semi-conductor surface α ≡ average tilt angle at which ions of halo dopantare implanted into semiconductor body relative to direction generallyperpendicular to upper semiconductor surface β ≡ average azimuthalangle, as measured in plane extending along upper semiconductor surface,at which ions of halo dopant impinge on photoresist mask relative tochannel length direction ΔV_(T) ≡ threshold voltage difference betweenlong-channel and short-channel threshold voltages ε_(GD) ≡ permittivityof gate dielectric material ε_(SC) ≡ permittivity of semiconductormaterial Φ_(F) ≡ Fermi potential (in volts) of semiconductor material inchannel zone Φ_(FPOLY) ≡ Fermi potential (in volts) at net dopantconcentration of gate electrode constituted with polycrystallinesemiconductor material Φ_(ND) ≡ work function (in volts) of gateelectrode Φ_(S) ≡ potential across depletion region along surface ofchannel zone Φ_(S0) ≡ work function (in volts) of semiconductor materialin channel zone η ≡ fitting parameter χ ≡ electron affinity ofsemiconductor material

Long-channel and short-channel n-channel IGFETs are respectivelyreferred to here, i.e., both below and above in the Brief Description ofthe Drawings, as long and short n-channel IGFETs. Similarly,long-channel and short-channel p-channel IGFETs are respectivelyreferred to here as long and short p-channel IGFETs. As used below, theterm “surface-adjoining” means adjoining (or extending to) the uppersemiconductor surface, i.e., the upper surface of a semiconductor bodyconsisting largely of monocrystalline, or largely monocrystalline,semiconductor material.

An IGFET is described below as symmetrical when it is configured inlargely a mirror-image manner along both of its source/drain zones andinto the intervening channel zone. For instance, an IGFET having aseparate halo pocket portion along each source/drain zone is typicallydescribed here as symmetrical provided that the source/drain zones are,except possibly for their lengths, largely mirror images of each other.However, due to factors such as partial shadowing during ionimplantation into the location of one of the halo pockets, the dopantprofiles in the halo pockets along the upper semiconductor surface maynot largely be mirror images. In such cases, there is typically someasymmetry in the IGFET's operation even though the IGFET may bedescribed as a symmetrical device.

In the following description, various situations are presented in whichan imaginary line (not shown) is described as (a) extending between thetwo source/drain zones, or between the intended locations for the twosource/drain zones, of an IGFET and thus across the IGFET's interveningchannel zone, or across the intended location for the channel zone, or(b) extending from the center length-wise of an IGFET and towards one oreach of the IGFET's two source/drain zones, or towards the intendedlocation for one or each of the two source/drain zones, and thus partlyacross the IGFET's channel zone, or partly across the intended locationfor the channel zone. Each such imaginary line, regardless of whether itis situated along or below the upper semiconductor surface, typicallyextends along the IGFET's channel length. Subject to any perturbationswhich may cause the upper surface of the channel zone to be non-planarand which may, or may not, affect the dopant distribution below thechannel zone's upper surface, each such imaginary line is typicallyapproximately straight.

There is no particular channel-length value which generally separatesthe short-channel and long-channel regimes of IGFET operation or whichgenerally distinguishes a short-channel IGFET from a long-channel IGFET.A short-channel IGFET, or an IGFET operating in the short-channelregime, is simply an IGFET whose characteristics are significantlyaffected by short-channel effects. A long-channel IGFET, or an IGFEToperating in the long-channel regime, is the converse of a short-channelIGFET. While the channel length value of approximately 0.4 μmconstitutes the boundary between the short-channel and long-channelregimes in the background art example of FIG. 1, thelong-channel/short-channel boundary can occur at a higher or lower valueof channel length depending on various factors such as gate dielectricthickness, minimum printable feature size, channel zone dopantconcentration, and body-source/drain junction depth.

Associated Device Physics

In the present invention, threshold voltage roll-off is alleviated andpunchthrough is inhibited at short channel length in multiple IGFETswhich are of different channel length and which are manufacturedaccording to the same, or largely the same, fabrication process in sucha way that the average net dopant concentration in the channel zones ofthe IGFETs increases appropriately with decreasing channel length. Anunderstanding of this phenomena begins with the following devicephysics.

The reference symbol “V_(T)” is used here to represent the generalthreshold voltage of an IGFET. “V_(T)” also represents the thresholdvoltage of an IGFET operating in the short-channel regime. The thresholdvoltage of an IGFET operating in the long-channel regime is denoted bythe symbol “V_(T0)”.

Long-channel threshold voltage V_(T0) for an n-channel IGFET is givenapproximately by the classical relationship: $\begin{matrix}{V_{T0} = {V_{FB} + \frac{q\quad t_{d{({INV})}}N_{C}}{C_{GD}} + \Phi_{S}}} & (1)\end{matrix}$

where V_(FB) is the flat-band voltage, q is the electronic charge,t_(d(INV)) is the thickness of the depletion region along the uppersurface of the channel zone at inversion, N_(C) is the average netdopant concentration in the IGFET's channel zone, C_(GD) is the gatedielectric capacitance per unit area, and Φ_(S) is the potential acrossthe channel surface depletion region at the onset of strong inversion.For Eq. 1, the IGFET lies along a surface, referred to here as the uppersemiconductor surface, of a doped monocrystalline semiconductor bodyhaving p-type body material that forms respective pn junctions with theIGFET's two source/drain zones. One of the source/drain zones acts asthe source (at any given time), while the other source/drain zone actsas the drain. The channel zone consists of the body-material portionlocated between the source/drain zones.

For an IGFET whose gate electrode consists of doped polycrystallinesemiconductor material, flat-band voltage V_(FB) is controlled by thedopant concentration in the gate electrode. Flat-band voltage V_(FB) isnegative for an enhancement-mode n-channel silicon-gate IGFET.

Surface depletion region thickness t_(d(INV)) at inversion is determinedapproximately from: $\begin{matrix}{t_{d{({INV})}} = \sqrt{\frac{2\quad ɛ_{SC}{\Phi_{S}}}{q\quad N_{C}}}} & (2)\end{matrix}$

where ∈_(SC) is the permittivity of the semiconductor material,typically silicon.

The value of long-channel threshold voltage V_(T0) is controlled by ionimplanting a suitable p-type dopant into the channel zone. Although thethreshold-adjust implant is generally most effective when the mean depthof the threshold-adjust implant into the channel zone is less thansurface depletion region thickness t_(d(INV)), the lowest implant energyavailable with conventional ion implantation equipment is typically sohigh that the mean depth of the threshold-adjust implant is oftengreater than depletion thickness t_(d(INV)) when channel length L isconsiderably less than 1 μm.

Surface depletion region potential Φ_(S) is twice the Fermi potentialΦ_(F) of the semiconductor material in the channel zone. That is,

Φ_(S)=2Φ_(F)  (3)

for which Fermi potential Φ_(F), negative for n-type semiconductormaterial and positive for p-type semiconductor material, is given from:$\begin{matrix}{{\Phi_{P}} = {\frac{kT}{q}{\ln \left( \frac{N_{C}}{n_{i}} \right)}}} & (4)\end{matrix}$

where k is Boltzmann's constant, T is temperature, and n_(i) is theintrinsic carrier concentration. Eqs. 1-4 indicate that long-channelthreshold voltage V_(T0) is a function of average net channel dopantconcentration N_(C). In attempting to scale an IGFET to shorter channellength, it is thus clear that threshold voltage V_(T0) cannot be scaledindependently of average channel concentration N_(C).

Flat-band voltage V_(FB) is determined from: $\begin{matrix}{V_{FB} = {\Phi_{M0} - \Phi_{S0} - \frac{Q_{SS}}{C_{GD}}}} & (5)\end{matrix}$

where Φ_(M0) is the work function of the gate electrode, Φ_(S0) is thework function of the semiconductor material at the upper surface of thechannel zone at the flat-band condition, and Q_(SS) is the charge perunit area at the interface between the gate dielectric and the channelzone. Assuming that the gate electrode consists of doped polycrystallinesemiconductor material of the same basic type, e.g., silicon, as theremainder of the IGFET, work functions Φ_(M0) and Φ_(S0) are given as:$\begin{matrix}{\Phi_{M0} = {\chi + \frac{E_{G}}{2} - \Phi_{FPOLY}}} & (6) \\{\Phi_{S0} = {\chi + \frac{E_{G}}{2} + \Phi_{F}}} & (7)\end{matrix}$

where Φ_(FPOLY) is the Fermi potential at the net dopant concentrationof the polycrystalline semiconductor material in the gate electrode, χis the electron affinity of the semiconductor material, and E_(G) is theband-gap energy of the semiconductor material. Polycrystallinegate-electrode Fermi potential Φ_(FPOLY) is negative for n-typepolycrystalline semiconductor material and positive for p-typepolycrystalline semiconductor material.

Combining Eqs. 1-7 yields the following approximate relationship forlong-channel threshold voltage V_(T0) for an n-channel enhancement-modeIGFET: $\begin{matrix}{V_{T0} = {{\frac{kT}{q}{\ln \left( \frac{N_{C}}{n_{i}} \right)}} + {\frac{2}{C_{GD}}\sqrt{N_{C}ɛ_{SC}{kT}\quad {\ln \left( \frac{N_{C}}{n_{i}} \right)}}} - \Phi_{FPOLY} - \frac{Q_{SS}}{C_{GD}}}} & (8)\end{matrix}$

When the polycrystalline semiconductor material is of the sameconductivity type, i.e., n-type, as the semiconductor material in thesource/drain zones, when the semiconductor material, including that ofthe gate electrode, is silicon, and when the gate dielectric is siliconoxide, semiconductor permittivity ∈_(SC) is 1.04×10⁻¹² F/cm,polycrystalline gate-electrode Fermi potential Φ_(FPOLY) is −0.57 eV,and areal interface charge Q_(SS) is approximately 10¹⁰ q per unit area.At a value of 5 nm for gate dielectric thickness t_(GD), areal gatedielectric capacitance C_(GD) is 6.91×10⁻⁷ F/cm². For these parametricvalues, FIG. 4 graphically illustrates how long-channel thresholdvoltage V_(T0) varies with average net channel dopant concentrationN_(C) at room temperature. Threshold voltage V_(T0) increases fromapproximately 0.1 V to approximately 0.9 V as average concentrationN_(C) in the channel zone is increased from 1×10¹⁷ atoms/cm³ to 1.4×10¹⁸atoms/cm³.

The threshold voltage V_(T) of a short-channel IGFET can be expressedas:

V _(T) =V _(T0) −ΔV _(T)  (9)

where threshold voltage difference ΔV_(T) is positive for an n-channelIGFET and negative for a p-channel IGFET. Using the model proposed byLiu et al, cited above, threshold voltage difference ΔV_(T) is given as:$\begin{matrix}{{\Delta \quad V_{T}} = \frac{{2\left( {V_{BI} - \Phi_{S}} \right)} + V_{{DS}{({TH})}}}{2\quad {\cosh \left( {{L/2}l} \right)}}} & (10)\end{matrix}$

where V_(BI) is the built-in voltage at the pn junction between the bodymaterial and the source/drain zone that acts as the source, V_(DS(TH))is the drain-to-source voltage at the threshold point, L is again thechannel length, and l is a normalization length. In other words,$\begin{matrix}{V_{T} = {V_{T0} - \frac{{2\left( {V_{BI} - \Phi_{S}} \right)} + V_{{DS}{({TH})}}}{2\quad {\cosh \left( {{L/2}l} \right)}}}} & (11)\end{matrix}$

in the model of Liu et al.

Built-in voltage V_(BI) for the junction between the body material andthe source/drain zone acting as the source is given as: $\begin{matrix}{V_{BI} = {\frac{kT}{q}{\ln \left( \frac{N_{C}N_{S/D}}{n_{i}^{2}} \right)}}} & (12)\end{matrix}$

where N_(S/D) is the average net dopant concentration in thesource/drain zones. Drain-to-source voltage V_(DS(TH)) at the thresholdpoint, as conventionally defined, is quite small, typically 0.1 V orless. Normalization length l is determined from: $\begin{matrix}{1 = \sqrt{\frac{ɛ_{SC}t_{GD}t_{d{({INV})}}}{ɛ_{GD}\eta}}} & (13)\end{matrix}$

where ∈_(GD) is the permittivity of the gate dielectric material,silicon oxide here, and η is a fitting parameter that accounts for thetwo-dimensional nature of an actual IGFET.

Following the line of approximation utilized in Liu et al, Eq. 10 isreduced to:

ΔV _(T)≈[2(V _(BI)−2Φ_(F))+V _(DS(TH)) ]e ^(−L/2l)  (14)

Using Eq. 9, threshold voltage V_(T) in the short-channel regime isgiven approximately as:

V _(T) ≈V _(T0)−[2(V _(BI)−2Φ_(F))+V _(DS(TH)) ]e ^(−L/2l)  (15)

where long-channel threshold voltage V_(T0) is given generally fromEq. 1. Combining Eqs. 2, 3, 13, and 15 yields: $\begin{matrix}{V_{T} = {V_{T0} - {\left\lbrack {{2\left( {V_{BI} - {2\Phi_{F}}} \right)} + V_{{DS}{({TH})}}} \right\rbrack {\exp \left( {{{- L}/2}\sqrt{\frac{2\quad ɛ_{SC}t_{GD}}{ɛ_{GD}\eta}\sqrt{\frac{ɛ_{SC}\Phi_{F}}{q\quad N_{C}}}}} \right)}}}} & (16)\end{matrix}$

where long-channel threshold voltage V_(T0) for an n-channelenhancement-mode IGFET is given specifically from Eq. 8, Fermi potentialΦ_(F) is given from Eq. 4. and built-in voltage V_(BI) is given from Eq.12. Eq. 15 or 16 reflects the experimentally observed exponential natureof short-channel threshold voltage roll-off.

Examination of Eq. 15 or 16 in light of the dependencies of Fermipotential Φ_(F) and built-in voltage V_(BI) on average net channeldopant concentration N_(C) as given by Eqs. 4 and 12 indicates thatincreasing average, concentration N_(C) in the channel zone causesthreshold voltage difference ΔV_(T) to decrease for typical values ofdrain-to-source voltage V_(DS(TH)) at the threshold point.

In particular, average net source/drain dopant concentration N_(S/D)exceeds average net channel dopant concentration N_(C). As averagechannel concentration N_(C) increases, the exponential term in Eq. 16thereby decreases faster than the [2(V_(BI)−2Φ_(F))+V_(DS(TH))] termincreases. Consequently, increasing average channel concentration N_(C)alleviates short-channel threshold voltage roll-off. That is, increasingaverage concentration N_(C) causes the onset of threshold voltageroll-off to be shifted to lower channel length.

As mentioned above, increasing average net channel dopant concentrationN_(C) causes long-channel threshold voltage V_(T0) to increase. In anIGFET fabrication process where the value of average channelconcentration N_(C) for a long-channel IGFET is approximately the sameas the value of average concentration N_(C) for a like-polarityshort-channel IGFET, the amount of V_(T0) increase that results fromincreasing concentration N_(C) to alleviate short-channel thresholdvoltage roll-off by a useful amount would be unsuitably high. Inquantitative terms, Eqs. 8 and 16 thus indicate that increasingconcentration N_(C) is not a suitable way for alleviating thresholdvoltage roll-off in a transistor fabrication process where concentrationN_(C) is approximately at the same value for long-channel andshort-channel IGFETs.

An example is helpful to clarify this point. Consider the situation inwhich average net channel dopant concentration N_(C) is 5×10¹⁷atoms/cm³, threshold voltage difference ΔV_(T) is 0.1 V, drain-to-sourcevoltage V_(DS(TH)) at the threshold point is 0.1 V, and the gatedielectric layer consists of silicon oxide for which gate dielectricthickness t_(GD) is approximately 5 nm. Long-channel threshold voltageV_(T0) is then approximately 0.45 V.

To reduce threshold voltage difference ΔV_(T) by a factor of 3 down toapproximately 0.03 V, average net channel dopant concentration N_(C)would need to increase by a factor of nearly 16. Average concentrationN_(C) in the channel zone would then roughly be 8×10¹⁸ atoms/cm³. Theresulting value of long-channel threshold voltage V_(T0) would roughlybe 2.4 V, a value which is unsuitably high compared to the V_(T0) valueof approximately 0.45 V that arises when average channel concentrationN_(C) is 5×10¹⁷ atoms/cm³.

A similar result occurs with p-channel IGFETs except that the polaritiesof the various voltages are largely reversed. In short, increasingaverage net channel dopant concentration N_(C) to alleviate thresholdvoltage roll-off in the situation where average channel concentrationN_(C) is approximately the same for both long-channel and short-channelIGFETs of the same polarity type leads to the magnitude of long-channelthreshold voltage V_(T0) becoming unsuitably large.

General Channel Doping Requirement for Alleviating Short-ChannelThreshold Voltage Roll-Off

The problem of the magnitude of long-channel threshold voltage V_(T0)being unsuitably high in the foregoing situation is, in accordance withthe invention, overcome by configuring multiple like-polarityenhancement-mode IGFETs manufactured according to the same, or largelythe same, fabrication process in such a way that average net channeldopant concentration N_(C) increases appropriately as channel length Ldecreases. Average channel concentration N_(C) for a long-channel IGFETis at a nominal value. Average channel concentration N_(C) for alike-polarity short-channel IGFET is at a value suitably greater thanthe nominal value so as to alleviate short-channel threshold voltageroll-off.

By appropriately controlling the variation of average net channel dopantconcentration N_(C) with channel length L, the threshold voltages of thelong-channel and short-channel IGFETs can be set at desired, typicallydifferent, values. The long-channel and short-channel IGFETs can beprovided in the same semiconductor structure and thus are manufacturedaccording to the same fabrication process, or can be provided inseparate semiconductor structures manufactured according to largely thesame fabrication process.

An important facet of the present invention is that average net channeldopant concentration N_(C) is a local, rather than global, parameter.More particularly, average channel concentration N_(C) for anyparticular implementation of certain prior art IGFET fabricationprocesses is the same, or largely the same, for all n-channel orp-channel IGFETs manufactured according to that process implementationdespite differences in channel length L. Although such a processimplementation can be modified to change average concentration N_(C),the new value of concentration N_(C) substantially applies to all IGFETsmanufactured according to the modified implementation of the prior artprocess regardless of the channel length. Because concentration N_(C) isthe same, or largely the same, for all IGFETs manufactured according toa particular implementation of any of these prior art fabricationprocesses, average channel concentration N_(C) is a global parameter inthese prior art processes.

In contrast, average net channel dopant concentration N_(C) forn-channel or p-channel IGFETs manufactured according to animplementation of the fabrication process of the invention variessignificantly with channel length L. This applies to both n-channel andp-channel IGFETs of different channel length in the same semiconductorstructure and to n-channel and p-channel IGFETs of different channellength in different semiconductor structures manufactured according tolargely the same fabrication process. When an implementation of thepresent process is modified to change average channel concentrationN_(C), the amount of change in average concentration N_(C) for an IGFETof one value of channel length L may, and typically does, differ fromthe amount of change in concentration N_(C) for a like-polarity IGFET ofanother value of channel length L. Since average concentration N_(C)varies with channel length L for n-channel or p-channel IGFETsmanufactured according to an implementation of the present fabricationprocess, average channel concentration N_(C) is a local parameter in theinvention.

FIG. 5 presents an example of how average net channel dopantconcentration N_(C) varies with channel length L and, in particular,increases suitably as channel length L decreases for n-channel andp-channel IGFETs manufactured according to an implementation of thepresent fabrication process so as to alleviate short-channel thresholdvoltage roll-off. The N_(C)(L) characteristics of FIG. 5 apply ton-channel and p-channel IGFETs whose gate dielectric thickness t_(GD) isapproximately 5 nm. As FIG. 5 indicates, average channel concentrationN_(C) increases from approximately 3.5×10¹⁷ atoms/cm³ at an L value of1.4 μm to approximately 9×10¹⁷ atoms/cm³ at an L value of 0.2 μm.

FIG. 6 illustrates how threshold voltage V_(T) varies with channellength L for n-channel IGFETs configured so that average net channeldopant concentration N_(C) decreases with increasing channel length L inthe manner shown in FIG. 5. The V_(T)(L) curve A labeled “Roll-offCompensating N_(C)(L)” in FIG. 6 corresponds to the N_(C)(L)characteristic of FIG. 5. For comparison purposes, FIG. 6 also depictssix examples of how threshold voltage V_(T) varies with channel length Lfor 5 nm-t_(GD) n-channel IGFETs for which average channel concentrationN_(C) is constant with channel length L. These comparative examples,collectively labeled “Constant N_(C)(L)”, are presented for N_(C) valuesvarying from 3.5×10¹⁷ atoms/cm³ to 1.1×10¹⁸ atoms/cm³, including anN_(C) value of 9×10¹⁷ atoms/cm³. Each of the six constant N_(C)(L)examples in FIG. 6 implements Eq. 15 or 16 presented above.

Examination of FIG. 6 shows that n-channel IGFETs manufactured accordingto a process implementation having the variable N_(C)(L) characteristicof FIG. 5 have much better short-channel threshold voltage stabilitythan the n-channel IGFETs having any of the constant values of averagenet channel dopant concentration N_(C). Roll-off compensating V_(T)(L)curve A of the invention reaches a maximum value V_(TAM) of thresholdvoltage V_(T) at an operating short-channel value L_(C) of channellength L. Operating short-channel value L_(C) is 0.2 μm in FIG. 6. Aschannel length L progressively drops below the short-channel L_(C) valueof 0.2 μm, threshold voltage V_(T) for roll-off compensating V_(T)(L)curve A decreases relatively softly at least down to an L value of 0.05μm. In contrast, threshold voltage V_(T) decreases very sharply aschannel length L progressively drops below 0.2 μm for each of the sixV_(T)(L) curves having constant values of average channel concentrationN_(C) in FIG. 6.

Particularly note the V_(T)(L) curve B having the constant N_(C) valueof 9×10¹⁷ atoms/cm³ in FIG. 6. This N_(C) value is the value of averagenet channel dopant concentration N_(C) at the 0.2-μm L_(C) value forroll-off compensating V_(T)(L) curve A. Threshold voltage V_(T) isapproximately 0.5 V at an L value of 0.2 μm for both curve A and curveB, the 0.5 V value of threshold voltage V_(T) being maximum valueV_(TAM) for roll-off compensating V_(T)(L) curve A. When channel lengthL drops from 0.2 μm to 0.1 μm, threshold voltage V_(T) drops by morethan 0.2 V for V_(T)(L) curve B at the constant N_(C) value of 9×10¹⁷atoms/cm³. Should fabrication variations result in a short n-channelIGFET having a channel length of 0.1 μm, the resulting V_(T) value ofless than 0.3 V is so low compared to the 0.5-V value at a channellength of 0.2 μm as to normally be unacceptable.

On the other hand, the V_(T) drop for roll-off compensating V_(T)(L)curve A is only 0.05 V when channel length L drops from 0.2 μm to 0.1μm. If fabrication variations result in a short n-channel IGFET having achannel length of 0.1 μm, the resulting V_(T) value is 0.45 V forroll-off compensating V_(T)(L) curve A. This V_(T) value is relativelyclose to the V?M value of 0.5 V and, importantly, would typically beacceptable.

In short, short-channel threshold voltage roll-off is significantlyalleviated in n-channel enhancement-mode IGFETs fabricated according toa process implementation that provides the inventive variable N_(C)(L)characteristic of FIG. 5 so as to achieve the roll-off compensatingV_(T)(L) characteristic of FIG. 6. The same occurs with p-channelenhancement-mode IGFETs except that V_(TAM) generally represents themagnitude (absolute value) of threshold voltage V_(T) at operatingshort-channel value L_(C) since threshold voltage V_(T) is negative forp-channel IGFETs. Consequently, the present invention increasestolerance to manufacturing variations.

As shown in FIG. 6, threshold voltage V_(T) for roll-off compensatingV_(T)(L) curve A drops slowly as channel length L increasesprogressively above the operating short-channel L_(C) value of 0.2 μmtowards the long-channel regime. A corresponding V_(T) change occurswith for p-channel IGFETs having the N_(C)(L) characteristic of FIG. 5except that threshold voltage V_(T) rises slowly, rather than droppingslowly, as channel length L increases progressively above the operatingshort-channel L_(C) value towards the long-channel regime. TheseV_(T)(L) characteristics for n-channel and p-channel IGFETs enablethreshold voltage V_(T) to be maintained within reasonable limits whenchannel length L changes unintentionally due to process variations.These V_(T)(L) characteristics also permit threshold voltage V_(T) to beadjusted by adjusting channel length L during IC design for L valuesgreater than L_(C).

Varying degrees of short-channel threshold voltage roll-off compensationcan be achieved by suitably modifying how average net channel dopantconcentration N_(C) increases with decreasing channel length. FIG. 7presents three examples of how average channel concentration N_(C)increases with decreasing channel length for 5 nm-t_(G) IGFETsmanufactured according to three corresponding implementations of thepresent fabrication process so as to alleviate threshold voltageroll-off. The middle N_(C)(L) curve shown in FIG. 7 repeats the N_(C)(L)curve shown in FIG. 5 and, as indicated by the label “Optimum N_(C)(L)”,provides approximately optimum threshold voltage roll-off compensation.The upper and lower N_(C)(L) curves in FIG. 7 respectively yieldover-compensation and under-compensation in reducing threshold voltageroll-off.

FIG. 8 illustrates how threshold voltage V_(T) varies with channellength L for n-channel IGFETs configured so that average net channeldopant concentration N_(C) varies with channel length L according toeach of the N_(C)(L) curves shown in FIG. 7. The thick-line curveslabeled “Over-Compensating N_(C)(L)”, “Optimum N_(C)(L)”, and“Under-Compensating N_(C)(L)” in FIG. 8 respectively correspond to thelike-labeled N_(C)(L) curves in FIG. 7. For comparison purposes, FIG. 8also depicts eight examples of how threshold voltage V_(T) varies withchannel length L for 5 nm-t_(GD) n-channel IGFETs for which averagechannel concentration N_(C) is constant with channel length L. Themiddle six of these eight constant N_(C)(L) curves in FIG. 8 repeat thesix constant N_(C)(L) curves in FIG. 6.

The V_(TAM) value for the under-compensating V_(T)(L) curve in FIG. 8 isless than the V_(TAM) value for the optimum V_(T)(L) curve and occurs ata higher L_(C) value than for the optimum V_(T)(L) curve. Accordingly,the under-compensating V_(T)(L) curve has a smaller reduction inthreshold voltage roll-off for channel length L in the vicinity of 0.2μm, the L_(c) value for the optimum V_(T)(L) curve. Somewhat theopposite occurs with the over-compensating V_(T)(L) curve in FIG. 8. TheV_(TAN) value for the over-compensating V_(T)(L) curve is greater thanthe V_(TAN) value for the optimum V_(T)(L) curve and occurs at a lowerL_(C) value than for the optimum V_(T)(L) curve. As a result, the effectof the optimum V_(T)(L) curve is to alleviate threshold voltage roll-offby shifting it to lower channel length while keeping threshold voltageV_(T) relatively high, especially in the vicinity of operatingshort-channel value L_(C). The derivative d_(VT)/d_(L) of thresholdvoltage V_(T) with channel length L is small in magnitude, preferablyzero, at value L_(C).

Threshold voltage V_(T) decreases with increasing channel length foreach of the three roll-off compensating V_(T)(L) curves in FIG. 8. Thegreatest V_(T) decrease with increasing channel length above L_(C)occurs with the over-compensating V_(T)(L) curve, and the lowest V_(T)decrease with increasing channel length above L_(C) occurs with theunder-compensating V_(T)(L) curve. For each of the three V_(T)(L)roll-off compensating curves, threshold voltage V_(T) is at least 0.03 Vless than maximum value V_(TAM) when channel length L is 0.3 μm greaterthan L_(C). Also, threshold voltage V_(T) is at least 0.1 V less thanvalue V_(TAM) for each of the three V_(T)(L) roll-off compensatingcurves when channel length L is 1.0 μm greater than L_(C).

As is evident from a comparison of the three V_(T)(L) roll-offcompensating curves in FIG. 8, the optimum V_(T)(L) curve provides afully adequate reduction in threshold voltage roll-off in the vicinityof a channel length of 0.2 μm while having an intermediate decrease inthreshold voltage V_(T) with increasing channel length above L_(C).Although this characteristic is typically optimum, the over-compensatingV_(T)(L) curve may be preferred in situations where unintentionalchannel-length variations caused by process variations generally resultin channel length L being on the low side of the nominal L value.

Structure of IGFETs Configured According to Invention

FIGS. 9a and 9 b respectively illustrate cross sections of a pair ofsymmetrical n-channel enhancement-mode IGFETs 40 and 42 manufacturedaccording to the same, or largely the same, fabrication process of theinvention so as to be of different channel length. IGFET 40 is along-channel device. IGFET 42 is a short-channel device. IGFETs 40 and42 are configured in accordance with the invention to implement theN_(C)(L) principles described in the previous section in order tosignificantly alleviate short-channel threshold voltage roll-off.

N-channel IGFETs 40 and 42 may be part of the same overall semiconductorstructure and thus are manufactured according to the same fabricationprocess. For simplicity, IGFETs 40 and 42 are treated here as being partof the same semiconductor structure in the following description ofFIGS. 9a and 9 b. Alternatively, IGFETs 40 and 42 may be parts ofseparately fabricated, and thus different, semiconductor structures. Inthis case, largely the same fabrication process is utilized tomanufacture IGFETs 40 and 42. In addition, the semiconductor structureor structures containing n-channel IGFETs 40 and 42 typically includesp-channel IGFETs configured in a similar, but complementary, manner toIGFETs 40 and 42.

For the indicated case in which n-channel IGFETs 40 and 42 are part ofthe same semiconductor structure, IGFETs 40 and 42 are created from amonocrystalline silicon (“monosilicon”) semiconductor body along whoseupper surface is situated a recessed field region 44 of electricallyinsulating material, typically primarily silicon oxide. Field-insulatingregion 44 laterally separates a group of active regions in themonosilicon semiconductor body. The semiconductor portions of IGFET 40are provided in one such active region 46. The semiconductor portions ofIGFET 42 are provided in another such active region 48.

The semiconductor body contains p-type body material 50. IGFETs 40 and42 may be formed from a single (continuous) region of p-type bodymaterial 50 or from a pair of laterally separated regions of bodymaterial 50. In the latter case, n-type material of the semiconductorbody normally separates the two regions of p-type body material 50 belowfield insulation 44. The bulk of body material 50 is lightly dopedp-type.

P-type body material 50 includes a pair of surface-adjoining upperportions 52 and 54 which respectively encompass parts of active regions46 and 48. Upper body-material portions 52 and 54 consist principally ofmoderately doped p-type semiconductor material (monosilicon). Asdiscussed further below, upper portions 52 and 54 include somesurface-adjoining heavily doped p-type semiconductor material. In theexample of FIGS. 9a and 9 b, upper portions 52 and 54 are illustrated asextending deeper into the semiconductive body than field insulation 44and thus respectively deeper into the semiconductor body than activeregions 46 and 48. Alternatively, field insulation 44 can extend deeperinto the semiconductor body than upper portions 52 and 54. As discussedfurther below, upper portions 52 and 54 are doped in such a way as toalleviate both threshold voltage roll-off and punchthrough.

Body material 50 also includes a pair of heavily doped well portions 56and 58. P+ well portion 56 is situated between upper body-materialportion 52 and the lightly doped bulk of body material 50. Similarly, p+well portion 58 is situated between upper body-material portion 54 andthe lightly doped bulk of body material 50. Although p+ wells 56 and 58are described here as separate portions of body material 50, they mayform a single, continuous p+ well.

Long n-channel IGFET 40 in FIG. 9a has a pair of composite n-typesource/drain zones 60 and 62 situated in active region 46 along theupper semiconductor surface. Source/drain zone 60 consists of a veryheavily doped main portion 60M and a more lightly doped extension 60E.Source/drain zone 62 consists of a very heavily doped main portion 62Mand a more lightly doped extension 62E. Although source/drain extensions60E and 62E are more lightly doped than n++ main source/drain portions60M and 62M, extensions 60E and 62E are still heavily doped n-type.

A channel zone 64 of p-type upper body-material portion 52 laterallyseparates source/drain zones 60 and 62. In particular, n+ source/drainextensions terminate channel zone 64 along the upper semiconductorsurface. A gate dielectric layer 66 lies on channel zone 64. A gateelectrode 68 consisting of very heavily doped n-type polycrystallinesilicon (“polysilicon”) lies on gate dielectric layer 66 above channelzone 64. Gate electrode 68 partially overlies source/drain extensions60E and 62E but normally does not extend above main source/drainportions 60M and 62M.

A pair of electrically insulating spacers 70 and 72 are situated alongthe opposite transverse sidewalls of gate electrode 68 respectivelyabove source/drain zones 60 and 62. Metal silicide layers 74, 76, and 78are respectively situated on components 68, 60M, and 62M. Silicidelayers 74, 76, and 78, typically consisting of cobalt silicide,facilitate making electrical contact to gate electrode 68 andsource/drain zones 60 and 62.

Short n-channel IGFET 40 in FIG. 9b has a pair of composite n-typesource/drain zones 80 and 82 situated in active region 48 along theupper semiconductor surface. Source/drain zone 80 consists of a veryheavily doped main portion 80M and a more lightly doped extension 80E.Source/drain zone 82 consists of a very heavily doped main portion 82Mand a more lightly doped extension 82E. Source/drain extensions 80E and82E, while being more lightly doped than n++ main source/drain portions80M and 82M, are heavily doped n-type. Source/drain zones 80 and 82,along with source/drain zones 60 and 62, extend to approximately thesame depth below the upper semiconductor surface.

A channel zone 84 of p-type upper body-material portion 54 laterallyseparates source/drain zones 80 and 82. Specifically, p+ source/drainextensions 80H and 82E terminate channel zone 84 along the uppersemiconductor surface. A gate dielectric layer 86 lies on channel zone84. A gate electrode 88 consisting of very heavily doped n-typepolysilicon lies on gate dielectric layer 86 above channel zone 84. Gateelectrode 88 partially overlies source/drain extensions 80E and 82E butnormally does not extend above main source/drain portions 80M and 82M.

A pair of electrically insulating spacers 90 and 92 are situated alongthe opposite transverse sidewalls of gate electrode 88 respectivelyabove source/drain zones 80 and 82. Metal silicide layers 94, 96, and 98are respectively situated on components 86, 80M, and 82M. Silicidelayers 94, 96, 98, typically consisting of cobalt silicide, facilitatemaking electrical contact to gate electrode 88 and source/drain zones 80and 82.

In IGFETs 40 and 42, gate sidewall spacers 70, 72, 90, and 92 may beformed with various electrical insulators. For instance, spacers 70, 72,90, and 92 may consist largely of silicon nitride, largely of siliconoxide, a combination of silicon nitride and silicon oxide, or acombination of either or both of these insulators with another insulatorsuch as tetraethyl orthosilicate (“TEOS”). In a typical implementation,each spacer 70, 72, 90, or 92 consists of a thin silicon oxide layer, athin overlying TEOS layer, and an overlying main silicon nitrideportion. Spacers 70, 72, 90, and 92 each have a bottom thickness of30-150 nm, typically 100 nm.

For simplicity and to maintain generality, the (potential) presence ofdifferent electrically insulating regions in any of gate sidewallspacers 70, 72, 90, and 92 is not indicated in the drawings. A singletype of shading is utilized for all of each of spacers 70, 72, 90, and92 in the drawings. The same applies to other such electricallyinsulating spacers shown in the drawings.

P-type channel zones 64 and 84 are configured in the following way.Channel zone 64 in long-channel IGFET 40 contains a pair of laterallyseparated heavily doped halo pocket portions 100 and 102 as indicated inFIG. 9a. P+ halo pocket portion 100 adjoins source/drain extension 60Ealong all of the pn-junction portion between extension 60E and channelzone 64. P+ halo pocket portion 102 similarly adjoins source/drainextension 62E along all of the pn junction portion between extension 62Eand channel zones 64. Halo pockets 100 and 102 extend respectively belowsource/drain extensions 60E and 62E to respectively meet mainsource/drain portions 60M and 62M. Pockets 100 and 102 achieve theclosest proximity to each other at, or close to, the upper semiconductorsurface.

P+ halo pocket portions 100 and 102 provide channel zone 64 with a netdopant concentration that reaches a local surface minimum along theupper semiconductor surface in the longitudinal direction, i.e., alongthe channel length, at a location between source/drain zones 60 and 62.An understanding of the shape of the dopant profile in channel zone 64along the upper semiconductor surface is facilitated with the assistanceof FIG. 10a which presents an example of how the net dopantconcentration N_(N) in long-channel IGFET 40 varies along the uppersemiconductor surface in the longitudinal direction. Horizontaldirection x in FIG. 10a is measured from the center lengthwise (channelcenter) of channel zone 64 along the upper semiconductor surfacelongitudinally towards either source/drain zone 60 or source/drain zone62. Curve segments 64*, 100*, 102*, 60E*, 62E*, 60M*, and 62M* in FIG.10a respectively represent the net dopant concentrations of components64, 100, 102, 60E, 62E, 60M, and 62M along the upper semiconductorsurface.

The-local surface minimum in the net dopant concentration of channelzone 64 in the longitudinal direction occurs in the upper semiconductorsurface portion situated between halo pocket portions 100 and 102. Asindicated by the flat portion of curve segment 64* in FIG. 10a, the netdopant concentration of channel zone 64 is approximately constant at thelongitudinal local surface minimum along a non-zero portion of animaginary line (not shown) extending longitudinally from the channelcenter along the upper semiconductor surface to each source/drain zone60 or 62. Hence, the net dopant concentration of channel zone 64 isapproximately constant at the longitudinal local surface minimum along anon-zero portion of a longer imaginary line (likewise, not shown)extending longitudinally between source/drain zones 60 and 62 along theupper semiconductor surface. The non-zero portion of the longer lineterminates at halo pockets 100 and 102. The flat non-haloed portion ofcurve segment 64* also indicates that an exemplary value for thelongitudinal local surface minimum in the net dopant concentration ofchannel zone 64 is 1×10¹⁷ atoms/cm³.

Part of curve segment 64* is also marked 100* or 102* in FIG. 10a. Asindicated by curve segment 100* or 102*, the net surface dopantconcentration of channel zone 64 increases in moving away from thelocation of the longitudinal local surface minimum longitudinallytowards source/drain zone 60 or 62 and longitudinally reaches a localsurface maximum at a location close to source/drain extension 60E or62E. Consequently, the surface dopant profile in channel zone 64 isshaped generally like a long saddle in the longitudinal direction. FIG.10a illustrates one half of the long saddle shape for channel zone 64.

P-type channel zone 84 in short-channel IGFET 42 contains a pair ofheavily doped halo pocket portions merged together to form a singleheavily doped composite halo pocket portion 104 that extends fullyacross channel zone 84. See FIG. 9b. P+ extended halo pocket portion 104adjoins source/drain extension 80E along all of the pn-junction portionbetween extension 80E and channel zone 84. Extended halo pocket portion104 likewise adjoins source/drain extension 82E along all of thepn-junction portion between extension 82E and channel zone 84. Extendedpocket 104 extends below source/drain extensions 80E and 82E torespectively meet main source/drain portions 80M and 82M.

Similar to what occurs in channel zone 64, p+ extended halo pocketportion 104 provides channel zone 84 with a net dopant concentrationthat reaches a local surface minimum along the upper semiconductorsurface in the longitudinal direction at a location between source/drainzones 80 and 82. An understanding of the shape of the dopant profile inchannel zone 84 along the upper semiconductor surface is facilitatedwith the assistance of FIG. 10b which presents an example of how the netdopant concentration N_(N) in short-channel IGFET 42 varies along theupper semiconductor surface in the longitudinal direction. Horizontaldistance x in FIG. 10b is measured from the center lengthwise (channelcenter) of channel zone 84 along the upper semiconductor surfacelongitudinally towards either source/drain zone 80 or source/drain zone82. Curve segments 84*, 104*, 80E*, 82E*, 80M*, and 82M* respectivelyrepresent the net dopant concentrations of components 84, 104, 80E, 82E,80M, and 82M along the upper semiconductor surface.

The local surface minimum in the net dopant concentration of channelzone 84 in the longitudinal direction occurs at approximately thechannel center of channel zone 84. Because composite halo pocket portion104 extends fully across channel zone 84, the net surface dopantconcentration of extended pocket portion 104, as represented by curvesegment 104* in FIG. 10b, is the same as the net surface dopantconcentration of channel zone 84, as represented by curve segment 84* inFIG. 10b. As indicated by curve segment 84* or 104* in FIG. 10b, thelongitudinal local surface minimum in the net dopant concentration ofchannel zone 84 is significantly greater than 1×10¹⁷ atoms/cm³. Anexemplary value for the longitudinal local surface minimum in the netdopant concentration of channel zone 84 is 4×10¹⁷ atoms/cm³. In anyevent, the longitudinal local surface minimum in the net dopantconcentration of channel zone 84 is greater than the correspondinglongitudinal local surface minimum in the net dopant concentration ofchannel zone 64.

The longitudinal local surface minimum in the net dopant concentrationof channel zone 84 can be described as a occurring approximately at thecenter point of an imaginary line (not shown) extending longitudinallybetween source/drain zones 80 and 82 along the upper semiconductorsurface. As indicated by curve portion 84* (or 104*) in FIG. 10b, thenet surface dopant concentration of channel zone 84 increases in movingaway from the point location of the longitudinal local surface minimumlongitudinally towards source/drain zone 80 or 82 so as tolongitudinally reach a local surface maximum at a location close tosource/drain extension 80E or 82E. Also, as described below inconnection with FIGS. 36 and 37, the longitudinal local surface maximumin the net dopant concentration of channel zone 84 is normally greaterthan the corresponding longitudinal local surface maximum in the netdopant concentration for channel zone 64.

Somewhat similar to the dopant profile in channel zone 64 oflong-channel IGFET 40, the surface dopant profile in channel zone 84 ofshort-channel IGFET 42 is shaped generally like a saddle in thelongitudinal direction. FIG. 10b illustrates one half of the saddleshape for channel zone 84.

Separate halo pocket portions 100 and 102 and extended halo pocketportion 104 are defined by p-type dopant referred to here, forconvenience, as the p-type halo dopant. As discussed below, the p-typehalo dopant is introduced into channel zones 64 and 84 in the samedoping operation.

The amount of p-type halo dopant utilized, per unit width of channelzone 84, to define extended halo pocket portion 104 is approximatelyequal to the amount of p-type halo dopant utilized, per unit width ofchannel zone 64, to define both of halo pocket portions 100 and 102.Accordingly, the surface dopant profiles in channel zones 84 and 64 aredefined by approximately equal amounts of halo dopant per unit channelwidth. The halo dopant utilized to define halo pockets 100 and 102 andextended pocket 104 increases average net dopant concentration N_(C) ineach of channel zones 64 and 84. Inasmuch as channel zone 84 is oflesser channel length than channel zone 64, channel zone 84 is ofgreater average net dopant concentration than channel zone 64.

In other words, the presence of separate halo pocket portions 100 and102 and extended halo pocket portion 104 causes average net channeldopant concentration N_(C) to increase as channel length L decreases. Bysuitably choosing the amount of p-type halo dopant utilized, per unitchannel width, to define halo pockets 100 and 102 and extended halopocket 104, an N_(C)(L) variation closely approximating the N_(C)(L)curves of FIGS. 5 and 7 is implemented. Accordingly, a V_(T)(L)variation closely approximating the threshold voltage roll-offcompensating V_(T)(L) curves of FIGS. 6 and 8 is implemented.Configuring halo pockets 100 and 102 and extended halo pocket 104 in theforegoing manner thereby alleviates short-channel threshold voltageroll-off. In addition, IGFET 42 which is of shorter channel length thanIGFET 40 and is thereby of greater average net channel dopantconcentration than IGFET 40 is of significantly greater thresholdvoltage than IGFET 40.

From another perspective, short-channel IGFET 42 and the channel surfacedopant profile of FIG. 10b are the respective end products oflong-channel IGFET 40 and the channel surface dopant profile of FIG. 10aas channel length L is reduced from the L value of channel zones 64 tothe L value of channel zones 84. Halo pocket portions 100 and 102 thenenable average net channel dopant concentration N_(C) to increase withdecreasing channel length for a given amount of p-type halo dopant, perunit channel width, producing halo pockets 100 and 102 and therebycausing the net dopant concentration in channel zones 64 to vary in theindicated saddle-shaped manner along the upper semiconductor surface inthe longitudinal direction.

Also, for a given amount of p-type halo dopant, per unit channel width,causing the net dopant concentration of channel zone 84 to vary in theindicated saddle-shaped manner along the upper semiconductor surface inthe longitudinal direction, the longitudinal local surface minimum ofthe net dopant concentration in channel zone 84 increases withdecreasing channel length. This starts to occur when channel length L ismade so small that halo pockets 100 and 102 in channel zone 64 justtouch each other and start to evolve into extended halo pocket 104 ofchannel zone 84.

Body material 50 in both long-channel IGFET 40 and short-channel IGFET42 is provided with a primary ion-implanted p-type dopant thatsubstantially reduces punchthrough. This ion-implanted dopant is forconvenience, referred to here as the p-type anti-punchthrough (again,“APT”) dopant even though it may do more than alleviate punchthrough.FIG. 11 illustrates an example of the vertical dopant profile for thep-type APT dopant. Vertical distance (or depth) y in FIG. 11 is measuredfrom the upper semiconductor surface downward.

The p-type APT dopant in FIG. 11 has a concentration N_(APT) thatnormally reaches a maximum value at a y depth of more than 0.1 μm belowthe upper semiconductor surface. The subsurface maximum in APT dopantconcentration N_(APT) normally occurs at a y depth of no more than 0.4μm. Depth y of the subsurface maximum in concentration N_(APT) ispreferably 0.15-0.35 μm, typically 0.2 μm as depicted in FIG. 11.

FIGS. 12a and 12 b respectively illustrate how the p-type APT dopantaffects the vertical dopant profiles in long n-channel IGFET 40 andshort n-channel IGFET 42. In particular, FIGS. 12a and 12 b depictexamples of how the net dopant concentration N_(B) in body material 50varies in the vertical direction along cross sections respectivelyextending through the channel centers of IGFETs 40 and 42. Verticaldistance (or depth) y in FIGS. 12a and 12 b is measured from the uppersemiconductor surface downward. Curve segments 64*, 52*, and 56* in FIG.12a respectively represent the net dopant concentrations of components64, 52, and 56 for IGFET 40. Curve segments 84*, 54*, and 58* in FIG.12b respectively represent the net dopant concentrations of components84, 54, and 58 for IGFET 42. Dotted curve segment 64* in FIG. 12brepeats solid curve segment 64* in FIG. 12a.

Upper body-material portion 52 for long-channel IGFET 40 is definedsubstantially by the combination of the p-type APT and halo dopants.Since neither of halo pocket portions 100 and 102 is present at thechannel center in IGFET 40, curve segment 52* at the channel centerclosely reflects the vertical dopant profile of the p-type APT dopant inFIG. 11. Along a vertical location where halo pocket 100 or 102 ispresent, the net dopant concentration of upper portion 52 in thevicinity of the upper semiconductor surface is elevated compared to whatis represented by curve segment 52* in FIG. 12a.

Upper body-material portion 54 for short-channel IGFET 42 is likewisedefined substantially by the combination of the p-type APT and halodopants. Because extended halo pocket portion 104 is present at thechannel center in IGFET 42, curve segment 54* at the channel center inthe vicinity of the upper semiconductor surface is elevated compared tothe vertical dopant profile for the p-type APT dopant in FIG. 11. Theamount of increased doping at the channel center is approximately theconcentration difference between curve segments 84* and 64* in FIG. 12b.As indicated by curve segments 84* and 64*, the amount of increaseddoping at the channel center for IGFET 42 increases from essentiallyzero at a location below the upper semiconductor surface to a maximum atthe upper semiconductor surface.

The p-type APT dopant is provided to p-type body material 50 in such away that the net dopant concentration N_(B) of body material 50 reachesa primary local subsurface maximum in each of upper body-materialportions 52 and 54 at a y depth of more than 0.1 μm below the uppersemiconductor surface but not more than 0.4 μm below the uppersemiconductor surface. Net body dopant concentration N_(B) for IGFET 40or 42 may be approximately constant at its primary local subsurfacemaximum along a non-zero portion of, and typically nearly all of, animaginary line (not shown) extending longitudinally from the channelcenter to each of source/drain zones 60 and 62 or 80 and 82 at the depthof the primary local subsurface maximum for that IGFET 40 or 42.

Alternatively, due to vertical spreading of the p-type halo dopant andgreater presence of the halo dopant close to source/drain zones 60 and62 or 80 and 82 than to the channel center, the primary local subsurfacemaximum in net body dopant concentration N_(B) for IGFET 40 or 42 mayoccur at a non-zero distance longitudinally away from the channelcenter, typically close to each of source/drain zones 60 and 62 or 80and 82. In either case, the depths of the primary local subsurfacemaxima in concentration N_(B) for IGFETs 40 and 42 are respectivelyapproximately the same as the depths at which the left-most humps ofcurve segments 52* and 54* in FIGS. 12a and 12 b reach their maximumvalues. Depth y of each primary local subsurface maximum inconcentration N_(B) is preferably 0.15-0.35 μm, typically 0.2 μm, foreach of IGFETs 40 and 42.

By having the p-type APT dopant reach maximum concentration more than0.1 μm below the upper semiconductor surface, the vertical dopantprofiles for channel zones 64 and 84 are of a soft retrograde nature.This is illustrated by curve segments 64* and 84* in FIGS. 12a and 12 b.Also, the longitudinal local surface minimum in the net dopantconcentration of channel zones 64 or 84 is less than the correspondingprimary local subsurface maximum in net body dopant concentration.

A depletion region (not shown) is induced in each of channel zones 64and 84 along the upper semiconductor surface. The net dopantconcentration in channel zones 64 and 84 is sufficiently high,especially along the upper semiconductor surface, that thicknesst_(d(INV)) of the surface depletion region at inversion for channel zone64 or 84 is normally less than 0.1 μm. For example, surface depletionthickness t_(d(INV)) is normally 0.04-0.06 μm, typically 0.05 μm.Accordingly, the primary local subsurface maximum in net body dopantconcentration N_(B) for each of IGFETs 40 and 42 normally occurs belowits surface depletion region.

The p-type dopant causes net body dopant concentration N_(B) to beconsiderably higher in channel zones 64 and 84 than in the bulk of bodymaterial 50. As indicated by curve segment 64* in FIG. 12a, the minimumvalue of the net dopant concentration in channel zone 64 is itslongitudinal local surface minimum, 1×10¹⁷ atoms/cm³ in the example ofFIG. 12a. The minimum value of the net dopant concentration in channelzones 84 is also its longitudinal local surface minimum and, asindicated by comparing curve segments 84* and 64* in FIG. 12b, isgreater than the corresponding longitudinal local surface minimum in thenet dopant concentration of channel zones 64. Hence, the longitudinalminimum value of the net dopant concentration in channel zones 64 and 84is the longitudinal local surface minimum of the net dopantconcentration in channel zones 64.

The longitudinal local surface minimum of the net dopant concentrationin channel zone 64 is normally at least 10 times greater, typically 50times greater, than the value of net dopant concentration N_(B) in thebulk of body material 50. A depletion region (not shown) extends alongthe pn junction between body material 50 and each of source/drain zones60, 62, 80, and 82. By utilizing the p-type APT dopant to so elevate thedoping in channel zones 64 and 84 and by positioning the p-type APTdopant for channel zones 64 and 84 in the manner described above, thethickness in the body-side portion of each of these body-to-source/draindepletion regions is considerably reduced at any given value ofdrain-to-source voltage. The APT dopant thereby significantly inhibitsthe source/drain zone 60 or 62 which acts as the drain (at anyparticular time) for IGFET 40 from punching through to the othersource/drain zone 62 or 60. The APT dopant likewise significantlyinhibits the drain-acting source/drain zone 80 or 82 for IGFET 42 frompunching through to the other source/drain zone 82 or 80.

P+ wells 56 and 58 are defined by a further ion-implanted p-type dopantprovided to body material 50. This further ion-implanted dopant isreferred to here, for convenience, as the p-type well dopant.Implantation of the p-type well dopant is accomplished in such a waythat net dopant concentration N_(B) of body material 50 reaches a pairof further local subsurface maxima at depths respectively below thedepths of the primary local subsurface maxima in net body dopantconcentration N_(B). See FIGS. 12a and 12 b. Depth y of the furtherlocal subsurface maxima in net body dopant concentration N_(B) isnormally 0.4-0.7 μm, typically 0.6 μm.

The further local subsurface maxima in net body dopant concentrationN_(B) for IGFETs 40 and 42 may occur away from their channel centers.However, the halo dopant typically has substantially no effect on bodyconcentration N_(B) at the depth of the further local subsurface maximain body concentration N_(B) for both of IGFETs 40 and 42. Accordingly,concentration N_(B) for IGFET 40 or 42 is typically approximatelyconstant at its further local subsurface maxima along a non-zero portionof, and typically nearly all of, an imaginary line (not shown) extendingfrom the channel center to each of source/drain zones 60 and 62 or 80and 82 at the depth of the further local subsurface maxima for IGFET 40or 42. The depth and magnitude of the further local subsurface maxima inconcentration N_(B), in combination with the vertical Gaussian-likespread in. the p-type well dopant, are normally of such a nature thatthe p-type well dopant has no significant effect on the net dopantconcentration along the upper semiconductor surface.

Net dopant concentration N_(B) in the lightly doped (p−) bulk of bodymaterial 50 is normally 3×10¹⁵-8×10¹⁵ atoms/cm³, typically 5×10¹⁵atoms/cm³. Each of the primary local subsurface maxima in net bodydopant concentration N_(B) is normally 2×10¹⁷-8×10¹⁷ atoms/cm³,typically 5×10¹⁷ atoms/cm³. Each of the further local subsurface maximain concentration N_(B) is normally 5×10¹⁷−1×10¹⁸ atoms/cm³, typically8×10¹⁷ atoms/cm³. Also, the local surface minimum in the net dopantconcentration of channel zone 64 is 5×10¹⁶-2×10¹⁷ atoms/cm³, typically1×10¹⁷ atoms/cm³.

Main source/drain portions 60M, 62M, 80M, and 82M, normally extend0.1-0.2 μm, typically 0.18 μm, below the upper semiconductor surface.The vertical dimensions of FETs 40 and 42 are typically chosen so thatmain portions 60M, 62M, 80M, and 82M extend to lesser depths below theupper semiconductor surface than both the primary and further localsubsurface maxima in net body dopant concentration N_(B).

Main source/drain portions 60M, 62M, 80M, and 82M reach their maximumnet dopant concentrations at, or close to (no more than 0.1 μm below),the upper semiconductor surface. The maximum net dopant concentration inmain portions 60M, 62M, 80M, and 82M is normally 2×10²⁰-2×10²¹atoms/cm³, typically 1×10²¹ atoms/cm³. Source/drain extensions 60E, 62E,80E, and 82E likewise reach their maximum net dopant concentrations at,or close to (no more than 0.05 μm below), the upper semiconductorsurface. The maximum net dopant concentration in extensions 60E, 62E,80E, and 82E is normally 5×10¹⁸-2×10¹⁹ atoms/cm³, typically 1×10¹⁹atoms/cm³.

Source/drain zones 60 and 62 of long-channel IGFET 40 are typicallyconfigured so that the pn junction between upper body-material portion52 and each of zones 60 and 62 has a “graded” characteristic. Moreparticularly, in moving upward from the junction between body-materialportion 52 and source/drain zone 60 or 62 along a vertical cross-sectionthrough the center lengthwise of main portion 60M or 62M of that zone 60or 62, the vertical dopant profile rises less sharply, on the average,to the profile's maximum value than what would occur if the n-typedopant which defines source/drain zone 60 or 62 along that cross-sectionthrough main portion 60M or 62M were ion implanted at largely a singleenergy. The graded-junction characteristic is achieved by providing eachsource/drain zone 60 or 62 with a lower portion which underlies, and ismore lightly doped than, main portion 60M or 62M. Source/drain zones 80and 82 of short-channel IGFET 42 are then typically configured in thesame way so that the pn junction between upper body-material portion 54and each of zones 80 and 82 is a graded junction.

By configuring source/drain zones 60, 62, 80, and 82 so as to havegraded-junction characteristics, the junction capacitance is reduced.Consequently, the switching speeds of IGFETs 40 and 42 are increased.For simplicity, the graded-junction features of source/drain zones 60,62, 80, and 82 are not shown in FIGS. 9a and 9 b or in the laterdrawings. Except for the p-channel graded-junction features shown inFIGS. 18a and 18 b discussed below, the graded-junction features ofother source/drain zones are likewise not shown in the drawings.

Thickness t_(GD) of each of gate dielectric layers 66 and 86 is normally2-10 nm, preferably 3-5 nm, typically 4 nm. Gate dielectric layers 66and 86 normally contain atoms of silicon and oxygen. With no otherelement being present to a significant degree in layers 66 and 86, thesilicon and oxygen form silicon oxide.

Gate dielectric layers 66 and 86 of n-channel IGFETs 40 and 42 aretypically formed at the same time as the gate dielectric layers ofp-channel IGFETs in the present semiconductor structure. When the gateelectrodes of the p-channel IGFETs are formed with boron-dopedpolysilicon, atoms of nitrogen may be incorporated into the gatedielectric layers of the p-channel IGFETs, and thus also into gatedielectric layers 66 and 86 of IGFETs 40 and 42, to inhibit boron in thegate electrodes of the p-channel IGFETs from penetrating into thechannel zones of the p-channel IGFETS. Gate dielectric layers 66 and 86of n-channel IGFETs 40 and 42, along with the gate dielectric layers ofthe p-channel IGFETS, then consist of silicon oxynitride.

Threshold voltage V_(T) of each of n-channel IGFETs 40 and 42 dependsspecifically on the average net dopant concentration in the IGFET'schannel surface depletion region. In particular, threshold voltage V_(T)increases as the average net dopant concentration in the surfacedepletion region increases, and vice versa. By setting IGFET 42 atshorter channel length than IGFET 40 so that the presence of separatehalo pocket portions 100 and 102 and extended halo pocket portion 104enables the local surface minimum in the net dopant concentration ofchannel zone 84 to be greater than the local surface minimum in the netdopant concentration of channel zone 64 and, consequently, so thatshort-channel IGFET 42 is of greater average net channel dopantconcentration than long-channel IGFET 40, the average net dopantconcentration in the surface depletion region of IGFET 42 is greaterthan the average net dopant concentration in the surface depletionregion of IGFET 40. Hence, short-channel IGFET 42 is of greaterthreshold voltage than long-channel IGFET 40.

The channel surface depletion regions of IGFETs 40 and 42 normallycontain parts of the p-type APT dopant. Consequently, threshold voltageV_(T) of IGFET 40 or 42 can normally be controlled by controlling thecharacteristics of the APT dopant. When the total dosage of the APTdopant is increased, the amount of APT dopant in each surface depletionregion is normally increased so as to increase the average net dopantconcentration in each surface depletion region. Since threshold voltageV_(T) increases with increasing average net dopant concentration in thesurface depletion region, threshold voltage V_(T) of each of IGFETs 40and 42 can be increased by increasing the total dosage of the APTdopant, and vice versa.

At any given value of channel length L, threshold voltage V_(T) forn-channel IGFET 40 or 42 typically increases approximately linearly withincreasing dosage of the p-type APT dopant for relatively smallvariations in the APT dosage. Threshold voltage V_(T) for IGFET 40 or 42also typically increases approximately linearly with increasing dosageof the p-type halo dopant for relatively small variations in the halodosage at any given value of channel length L. Consequently, thresholdvoltage V_(T) for IGFET 40 or 42 at any given value of channel length Lis described approximately by the following relationship for relativelysmall variations in the APT and halo dosages:

V _(T) =a·DOSE_(Halo) +b·DOSE_(APT)  (17)

where DOSE_(Halo) is the dosage of the halo dopant, DOSE_(APT) is thedosage of the APT dopant, and a and b are coefficients dependent onchannel length L. Coefficients a and b are positive for n-channel IGFET40 or 42.

Coefficients a and b are determined experimentally at each channellength value of interest for a semiconductor fabrication process whosecharacteristics are fixed aside from variations in the halo and APTdosages. While determining coefficients a and b can be laborious, Eq. 17shows that a desired value of threshold voltage V_(T) at any given valueof channel length L can be achieved by suitably adjusting the halodosage or/and the APT dosage.

The head of a distribution of semiconductor dopant ion implanted througha specified surface of a semiconductor body and into the body consistsof the portion of the implanted dopant situated between the specifiedsurface of the body and the location at which the dopant reaches maximumconcentration. For a dopant implanted through the upper semiconductorsurface, the head of the dopant is therefore the upper portion of thedopant. As mentioned above, the primary local subsurface maxima in netdopant concentration N_(B) of body material 50 normally occurrespectively below the surface depletion regions. Since the primarylocal subsurface maxima in net body dopant concentration N_(B) areformed by the p-type APT dopant, substantially only part of the head ofthe APT implant is normally present in each channel surface depletionregion.

The p-type APT dopant is introduced into the semiconductor body by ionimplantation. Increasing the implantation energy causes the primarylocal subsurface maxima in net body dopant concentration N_(B) forIGFETs 40 and 42 to occur at greater depth below the upper semiconductorsurface so that the length of the head of the APT dopant is increased.As the head of the APT dopant becomes longer, less of the APT dopantaccumulates in each channel surface depletion region. This reduces theaverage net dopant concentration in each surface depletion region. Sincethreshold voltage V_(T) decreases with decreasing average net dopantconcentration in the associated surface depletion region, thresholdvoltage V_(T) of each of IGFETs 40 and 42 can be decreased by increasingthe implantation energy of the APT dopant so as to increase the depth atwhich the primary local subsurface maxima in net body dopantconcentration N_(B) occurs below the upper semiconductor surface, andvice versa.

As further mentioned above, the implanted p-type well dopant whichcreates the further local subsurface maxima in net body dopantconcentration N_(B) normally has no significant effect on the net dopantconcentration along the upper semiconductor surface. More particularly,the amount of well dopant present in the channel surface depletionregions is normally insignificant compared to the amount of implantedp-type APT dopant present in the surface depletion regions. Accordingly,the p-type well dopant normally has no significant effect on thresholdvoltage V_(T) of IGFET 40 or 42.

N-channel IGFETs 40 and 42 can be modified by providing body material 50with a third ion-implanted p-type dopant to flatten the vertical dopantprofile between the primary (APT) and further (well) local subsurfacemaxima in net body dopant concentration N_(B) for each of IGFETs 40 and42 and thereby bolster the performance characteristics by eliminatingweak areas where current leakage can occur, including well-to-wellleakage. This third dopant is referred to here, for convenience, as theisolation dopant because it helps isolate p-wells 56 and 58 from anadjacent n-type well so as to avoid well-to-well leakage.

FIGS. 13a and 13 b respectively illustrate a symmetrical long n-channelIGFET 110 and a symmetrical short n-channel IGFET 112 created from amonosilicon semiconductor body furnished with the p-type isolationdopant along with the p-type APT and well dopants. Except for theisolation dopant, IGFETs 110 and 112 are respectively identical toIGFETs 40 and 42. Accordingly, short-channel threshold voltage roll-offis alleviated in IGFETs 110 and 112 in substantially the same way as inIGFETs 40 and 42.

Body material 50 in n-channel IGFETs 110 and 112 contains a pair ofheavily doped intermediate portions 114 and 116 defined by the p-typeisolation dopant. P+ intermediate portion 114 lies between upperbody-material portion 52 and well 56 and extends partly into activeregion 46 for IGFET 110. P+ intermediate portion 116 lies between upperbody-material portion 54 and well 58 and extends partly into activeregion 48 for IGFET 112. Although intermediate portions 114 and 116 aredescribed here as heavily doped, they can also be described asmoderately doped.

FIGS. 14a and 14 b depict how the p-type isolation dopant affects thevertical dopant profiles along cross sections respectively extendingthrough the channel centers in n-channel IGFETs 110 and 112. Aside fromthis, FIGS. 14a and 14 b largely respectively repeat FIGS. 12a and 12 b.Curve segment 114* in FIG. 14a represents the net dopant concentrationof intermediate body-material portion 114. Curve segment 116* in FIG.14b represents the net dopant concentration of intermediatebody-material portion 116.

Intermediate body-material portions 114 and 116 are defined by providingthe semiconductor body with the p-type isolation dopant in such a waythat net dopant concentration N_(B) of body material 50 reaches a pairof additional local subsurface maxima at depths respectively below thedepths of the primary subsurface maxima in net body concentration N_(B)but respectively above the depths of the further local subsurface maximain body concentration N_(B). Depth y of the additional local subsurfacemaxima in concentration N_(B) is normally 0.3-0.5 μm, typically 0.4 μm,below the upper semiconductor surface. Each of the additional localsubsurface maxima in concentration N_(B) is normally 2×10¹⁷-8×10¹⁷atoms/cm³, typically 5×10¹⁷ atoms/cm³.

The semiconductor structure containing long n-channel IGFET 40 and/orshort n-channel IGFET 42, or long n-channel IGFET 110 and/or shortn-channel IGFET 112, normally also contains a long p-channel IGFETand/or a short p-channel IGFET. The p-channel IGFETs typically are, butneed not be, configured analogously to n-channel IGFETs 40 and 42, orn-channel IGFETs 110 and 112, for alleviating short-channel thresholdvoltage roll-off and avoiding punchthrough in accordance with theinvention.

FIGS. 15a and 15 b respectively illustrate a symmetrical long p-channelIGFET 120 and a symmetrical short p-channel IGFET 122 so configuredaccording to the invention. Just as the semiconductor structurecontaining one or more n-channel IGFETs configured according to theinvention need not contain any p-channel IGFET configured according tothe invention, the semiconductor structure containing long p-channelIGFET 120 and/or short p-channel IGFET 122 need not contain anyn-channel IGFET configured according to the invention.

P-channel IGFETs 120 and 122 may be part of the same overallsemiconductor structure and thus are manufactured according to the samefabrication process. For simplicity, p-channel IGFETs 120 and 122 aretreated here as being part of the same semiconductor structure in thefollowing description of FIGS. 15a and 15 b. Alternatively, IGFETs 120and 122 may be part of separately fabricated, and thus different,semiconductor structures. In this case, largely the same fabricationprocess is employed to manufacture IGFETs 120 and 122.

For the indicated case in which p-channel IGFETs 120 and 122 are part ofthe same semiconductor structure, the active semiconductor portions ofIGFETs 120 and 122 are respectively formed in active regions 126 and 128laterally surrounded by field-insulating region 44. The semiconductorbody utilized for creating IGFETs 120 and 122 contains n-type bodymaterial 130 situated over the lightly doped bulk of p-type bodymaterial 50. IGFETs 120 and 122 may be formed from a single region ofn-type body material 130 or from a pair of laterally separated regionsof body material 130. In the latter case, part of p-type body materialSo normally laterally separates the two regions of n-type body material130 below field insulation 44.

N-type body material 130 includes a pair of upper portions 132 and 134respectively analogous to, and doped in a complementary way, to upperportions 52 and 54 of p-type body material 50. N-type body material 130also includes a pair of heavily doped well portions 136 and 138respectively analogous to p+ well portions 56 and 58 of body material 50

P-channel IGFETs 120 and 122 contain source/drain zones (includingsource/drain extensions and graded-junction features), channel zones, apair of laterally separated halo pocket portions in the channel zone oflong-channel IGFET 120, a pair of halo pocket portions merged togetherto form a composite extended halo pocket portion in the channel zone ofshort-channel IGFET 122, gate dielectric layers, doped polysilicon gateelectrodes, gate sidewall spacers, and metal silicide layers configuredthe same as the corresponding elements in n-channel IGFETs 40 and 42 butindicated by reference symbols 80 higher in FIGS. 15a and 15 b. Each ofthe semiconductor regions in p-channel IGFETs 120 and 122 is of the samerelative dopant concentration level, but of opposite conductivity type,to the corresponding semiconductor region in n-channel IGFETs 40 and 42.Pn junction depths are typically slightly greater in p-channel IGFETs120 and 122 than in n-channel IGFETs 40 and 42. Subject to thesecomments, short-channel threshold voltage roll-off and punchthrough arealleviated in p-channel IGFETs 120 and 122 in substantially the same wayas in n-channel IGFETs 40 and 42.

Separate halo pocket portions 180 and 182 for long p-channel IGFET 120and extended halo pocket portion 184 for short p-channel IGFET 122 aredefined by n-type halo dopant analogous to the p-type halo dopant usedfor n-channel IGFETs 40 and 42. N-type upper body-material portions 132and 134 for p-channel IGFETs 120 and 122 are defined by the combinationof the n-type halo dopant and an ion-implanted n-type APT dopantanalogous to the p-type APT dopant used for n-channel IGFETs 40 and 42.P-channel IGFETs 120 and 122 may also be modified analogous to the wayin which n-channel IGFETs 40 and 42 are modified to produce n-channelIGFETs 110 and 112.

Threshold voltage V_(T) for a p-channel IGFET of the invention can becontrolled in the same way utilized to control threshold voltage V_(T)for an n-channel IGFET of the invention. For example, threshold voltageV_(T) of p-channel IGFET 120 or 122 can be controlled by adjusting theimplantation energy of the n-type APT dopant. At any given value ofchannel length L, threshold voltage V_(T) of IGFET 120 or 122 thussatisfies Eq. 17 for small variations in the APT and halo dosagessubject to coefficients a and b being negative here.

Fabrication of IGFET Structure According to Invention

FIGS. 16a-16 m (collectively “FIG. 16”) illustrate a complementary-IGFETprocess in accordance with the invention for manufacturing n-channelIGFETs 40 and 42 and p-channel IGFETs 120 and 122. Only the fabricationof short-channel IGFETs 42 and 122 is, for simplicity, actually depictedin FIG. 16. Long-channel IGFETs 40 and 120 are manufactured according tothe same fabrication steps utilized to manufacture short-channel IGFETs42 and 122. The only difference in fabrication is that IGFETs 40 and 120are respectively laid out to be of greater channel length than IGFETs 42and 122. To help clarify the fabrication of long-channel IGFETs 40 and120, reference symbols particular to the manufacture of IGFETs 40 and120 are generally indicated parenthetically after the respectivereference symbols for short-channel IGFETs 42 and 122 in the fabricationprocess described below.

Although the semiconductor structure created according to the presentcomplementary-IGFET fabrication process is described below as having allof IGFETs 40, 42, 120, and 122, the semiconductor structure may haveonly one of n-channel IGFETs 40 and 42 and/or only one of p-channelIGFETs 120 and 122. Any of IGFETs 40, 42, 120, and 122 not created inthe semiconductor structure described below are normally created in oneor more other semiconductor structures fabricated according to largelythe same fabrication steps as utilized in the below-describedfabrication process. Also, the fabrication steps unique to the creationof p-channel IGFETs 120 and 122 can be deleted from the fabricationprocess so that the semiconductor structure only contains n-channelIGFET 40 and/or n-channel IGFET 42, and vice versa.

Except for the p-type and n-type halo ion implantation steps, all of theion implantation steps in the present fabrication process are performedroughly perpendicular to the lower semiconductor surface and thusroughly perpendicular to the upper semiconductor surface. Moreparticularly, all of the non-halo implantation steps are performed at aselected angle, typically 7°, to the vertical. This small deviation fromperpendicularity for the non-halo implantation steps is used to avoidundesirable ion channeling effects. For simplicity, the small non-halodeviation from perpendicularity is not indicated in FIG. 16.

Unless otherwise indicated, the species of n-type dopant utilized ineach of the n-type ion implantations in the fabrication process of FIG.16 consists of the specified n-type dopant in elemental form. That is,each n-type ion implantation is performed with ions of the specifiedn-type dopant element rather than with ions of a chemical compound thatcontains the n-type dopant. The species of p-type dopant employed ineach of the p-type ion implantations variously consists of the p-typedopant, normally boron, in elemental or compound form. Hence, eachp-type ion implantation is normally performed with boron ions or withions of a boron-containing compound.

The starting point for the fabrication process of FIG. 16 is amonocrystalline semiconductor body consisting of a heavily doped p-typesubstrate 200 and an overlying lightly doped p-type epitaxial layer 50P.See FIG. 16a. P+ substrate 200 is a semiconductor wafer formed with<100> monosilicon doped with boron to achieve a typical resistivity of0.015 ohm-cm. For simplicity, substrate 200 is not shown in theremainder of FIG. 16. Alternatively, the starting point can simply be alightly doped p-type substrate.

P− epitaxial layer 50P consists of epitaxially grown <100> monosilicondoped with boron to achieve a typical resistivity of 30 ohm-cmcorresponding to the typical value, 5×10¹⁵ atoms/cm³, for net dopantconcentration N_(B) in the bulk of p-type body material 50. Epitaxiallayer 50P is a precursor to body material 50, the letter “P” at the endof a reference symbol being utilized here to indicate a precursor to aregion identified by the portion of the reference symbol preceding theletter “P”. The thickness of epitaxial layer 50P is typically 5.5 μm.

Field-insulating region 44 is provided along the upper surface of p−epitaxial layer 50P so as to define active regions 48 (46) and 128 (126)for IGFETs 42 (40) and 122 (120). See FIG. 16b. Field insulation 44 ispreferably created according to a trench-oxide technique such as thatdescribed in Wang, U.S. patent application Ser. No. 09/211,703, filed 14Dec. 1998, now U.S. Pat. No. 6,461,932 B1, the contents of which areincorporated by reference. In this case, field insulation 44 typicallyextends 0.35 μm into epitaxial layer 50P. Field insulation 44 may alsobe created according to a local-oxidation technique. In providing fieldinsulation 44, a thin screen insulating layer 202 of silicon oxide isthermally grown along the upper surface of epitaxial layer 50P.Screen-oxide layer 202 is typically 15 nm in thickness.

A photoresist mask 204 is formed on screen-oxide layer 202 above activeregion 48 (46) as indicated in FIG. 16c. Precursor n-type body material130P is formed in p− epitaxial layer 50P by introducing n-type dopantthrough the section of screen oxide 202 not covered by photoresist 204and into the underlying semiconductor material. Body material 130Pconsists of a moderately doped n-type surface-adjoining precursor upperportion 134P (132P) and an underlying heavily doped n-type well portion138 (136).

N+ well 138 (136) is defined by ion implanting a species of an n-typewell dopant at a heavy dosage through the uncovered section of screenoxide 202 and into epitaxial layer 50P. The dosage of the n-type welldopant is 5×10¹²-4×10¹³ ions/cm², typically 2×10¹³ ions/cm². The n-typewell dopant may consist of phosphorus or arsenic. The implantationenergy for the n-type well dopant is typically selected so that, in thefinal semiconductor structure, the n-type well dopant has no significanteffect on the average net dopant concentration along the uppersemiconductor surface, and, in particular, has no significant effect onthreshold voltage V_(T) of p-channel IGFET 122 (120). For the typicalcase in which phosphorus constitutes the n-type well dopant, theimplantation energy is 400-600 KeV, typically 500 KeV.

N upper body-material portion 134P (132P) is defined by introducing aspecies of an n-type dopant at a moderate dosage through the uncoveredsection of screen-oxide layer 202 and into epitaxial layer 50P. Thisn-type dopant serves, at least, to alleviate punchthrough and is, forconvenience, referred to as the n-type APT dopant. Introduction of then-type APT dopant into the semiconductor body is performed by ionimplantation. The dosage of the n-type APT dopant is 1×10¹²-1×10¹³ions/cm², typically 3×10¹² ions/cm². The n-type APT dopant may consistof phosphorus or arsenic.

In the final semiconductor structure, the n-type APT dopant has avertical dopant profile similar to that shown in FIG. 11 for the p-typeAPT dopant. The implantation energy for the n-type APT dopant isselected so that, after gate electrode 168 (148) is created forp-channel IGFET 122 (120), the n-type APT dopant reaches a maximumconcentration in upper body-material portion 134 (132) at a depth ofmore than 0.1 μm below the upper semiconductor surface but not more than0.4 μm below the upper semiconductor surface. The depth of the maximumconcentration of the n-type APT dopant is preferably 0.15-0.35 μm,typically 0.2 μm, below the upper semiconductor surface. For the typicalcase in which phosphorus constitutes the n-type APT dopant, theimplantation energy is 120-140 KeV, typically 130 KeV.

The n-type APT dopant causes net dopant concentration N_(B) of n-typebody material 130 in the final semiconductor structure to reach aprimary local subsurface maximum in n-type upper body-material portion134 at a depth of more than 0.1 μm below the upper semiconductor surfacebut not more than 0.4 μm below the upper semiconductor surface. Thedepth of the primary local subsurface maximum in concentration N_(B) ispreferably 0.15-0.35 μm, typically 0.2 μm, below the upper semiconductorsurface. Also, the depth of the primary local subsurface maximum inconcentration N_(B) for p-channel IGFET 122 (120) is normally greaterthan the thickness of its channel surface depletion region.

Similar to what was said above about the p-type APT dopant,substantially only part of the head of the n-type APT dopant is presentin the surface depletion region of p-channel IGFET 122 (120). Themagnitude of threshold voltage V_(T) of IGFET 122 (120) can thereby bedecreased largely independent of channel length by increasing theimplantation energy of the n-type APT dopant so as to increase the depthat which the primary local subsurface N_(B) maximum occurs in upperbody-material portion 134 (132), and vice versa.

Implantation of the n-type APT dopant can be performed before implantingthe n-type well dopant. Also, if the semiconductor structure is to beprovided with an ion-implanted n-type isolation dopant analogous to thep-type isolation dopant described above, the n-type isolation dopant ision implanted through the uncovered section of screen oxide 202 whilephotoresist 204 is in place. The n-type isolation dopant may consist ofphosphorus or arsenic. When phosphorus constitutes the n-type isolationdopant, the implantation energy is typically 230 KeV. The n-typeisolation implant can be performed as the first, second, or third of thethree n-type implantations. Photoresist 204 is removed after performingthese two or three n-type implantations.

A photoresist mask 206 is formed on screen oxide 202 above active region128 (126). See FIG. 16d. P-type dopant is introduced through the sectionof screen oxide 202 not covered by photoresist 206 and into theunderlying semiconductor material to provide epitaxial layer 50P with amoderately doped p-type surface-adjoining precursor upper portion 54P(52P) and a heavily doped p-type well portion 58 (56).

P+ well 58 (56) is defined by ion implanting a species of theabove-mentioned p-type well dopant at a heavy dosage through theuncovered section of screen oxide 202. The dosage of the p-type welldopant is 1×10¹³-3×10¹³ ions/cm², typically 2×10¹³ ions/cm². The p-typewell dopant may consist of boron in elemental form or in the form ofboron difluoride. The implantation energy for the p-type well dopant istypically selected so that, in the final semiconductor structure, thep-type well dopant does not significantly affect threshold voltage V_(T)of n-channel IGFET 42 (40). For the typical case in which the p-typewell dopant consists of elemental boron, the implantation energy is200-220 KeV, typically 210 KeV.

P upper body-material portion 54P (52P) is defined by ion implanting aspecies of the above-mentioned p-type APT dopant at a moderate dosagethrough the uncovered section of screen oxide 202. The dosage of thep-type APT dopant is 2×10¹²-1×10¹³ ions/cm², typically 5×10¹² ions/cm².The p-type APT dopant may consist of boron in elemental form or in theform of boron difluoride.

FIG. 11 illustrates the typical vertical dopant profile for the p-typeAPT dopant in the final semiconductor structure. The implantation energyfor the p-type APT dopant is selected so that, after gate electrode 88(68) is created for n-channel IGFET 42 (40), the p-type APT dopantreaches a maximum concentration in upper body-material portion 54 (52)at a depth of more than 0.1 μm below the upper semiconductor surface butnot more than 0.4 μm below the upper semiconductor surface. For thetypical case in which the p-type APT dopant consists of elemental boron,the implantation energy is 40-70 KeV, typically 60 KeV.

Implantation of the p-type APT dopant can be performed before implantingthe p-type well dopant. If the semiconductor structure is to be providedwith intermediate p-type body-material portion 116 (114). theabove-mentioned p-type isolation dopant for intermediate portion 116(114) is implanted through the. uncovered section of screen oxide 202while photoresist 206 is in place. The p-type isolation dopant normallyconsists of elemental boron implanted at an energy of 110 KeV. Thep-type isolation dopant can be performed as the first, second, or thirdof the three p-type implantations. Photoresist 206 is removed afterperforming these two or three p-type implantations.

A thermal anneal is optionally performed on the resultant semiconductorstructure to repair lattice damage and place the implanted n-type andp-type dopants in energetically more stable sites so as to minimizefurther diffusion of these dopants during subsequent thermal operations,especially the final anneal described below. This optional anneal,referred to here as the well-APT anneal, is preferably a rapid thermalanneal (“RTAN”) at 1000-1100° C., typically 1075° C., for 5-20 sec.,typically 10 sec. The RTA is normally performed in a non-reactiveenvironment, typically nitrogen, but can be done in a reactive, e.g.,oxidizing, environment. By using the RTA, the implanted dopants areactivated without significant dopant movement (redistribution). Althoughthe above-mentioned implantation steps define precursor upperbody-material portions 54P (52P) and 134P (132P) and wells 58 (56) and138 (136), the annealing step completes the formation of these regions.

Subsequent to the well-APT anneal, screen oxide 202 is removed, and theupper semiconductor surface is cleaned. A composite gate dielectriclayer, portions of which constitute gate dielectric layers 86 (66) and166 (146), is provided along the upper semiconductor surface to athickness of 2-10 nm. See FIG. 16e. The composite gate dielectric layeris normally thermally grown. To prevent redistribution of the dopants inupper body-material portions 54P (52P) and 134P (132P) and wells 58 (56)and 138 (136), the thermal growth of the composite gate dielectric isusually performed in a “wet” environment according to a thermal profilein which the maximum thermal-oxidation temperature is relatively low,typically 700-900° C.

The thermally grown composite gate dielectric layer typically consistsof silicon oxide for which the wet environment is formed with oxygen andhydrogen. The thickness of the silicon oxide is normally 3-5 nm,typically 4 nm. Alternatively, in thermally growing the composite gatedielectric layer, atoms of nitrogen may be incorporated into thecomposite gate dielectric by providing oxygen from a gas, such asnitrous oxide or nitric oxide, containing nitrogen in a reactive form inorder to prevent boron in gate electrode 168 (148) of p-channel IGFET122 (120) from diffusing into channel zone 164 (144). The thickness ofthe resulting silicon oxynitride is likewise normally 3-5 nm, typically4 nm.

FIG. 17 illustrates a typical thermal schedule for growing the compositegate dielectric layer according to a low-temperature wet oxidationtechnique, including the optional incorporation of nitrogen into thegate dielectric. The thermal schedule in FIG. 17 consists of a ramp-upfrom 700° C. to 750° C. at 10° C./min. in nitrogen with 1% oxygen, astabilization at 750° C. for 10 min. in nitrogen with 1% oxygen, athermal oxidation at 750° C. for 3-6 min., typically 5 min., in hydrogenand oxygen diluted 90% in argon, a stabilization at 750° C. for 10 min.in nitrogen, a ramp-up from 750° C. to 900° C. at 10° C./min. innitrogen, an anneal at 900° C. for 10-15 min., typically 10 min., innitrogen or nitrous oxide, and a ramp down from 900° C. to 600° C. innitrogen. The resulting gate dielectric thickness is typically targetedto be 4 nm.

The anneal at 900° C. in nitrogen or nitrous oxide determines whetherthe composite gate dielectric layer consists of silicon oxide or siliconoxynitride. When the 900° C. anneal is done is nitrogen, the gatedielectric layer consists substantially of silicon oxide. The gatedielectric layer consists of silicon oxynitride when the 900° C. annealis done in nitrous oxide. This occurs because nitrous oxide is much morereactive than nitrogen and results in nitrogen being introduced into thegate dielectric.

Returning to FIG. 16e, a layer of largely undoped (intrinsic)polysilicon is deposited on top of the composite gate dielectric layerto a thickness of 200-300 nm, typically 250 nm. Utilizing a suitablephotoresist mask (not shown), the polysilicon layer is patterned toproduce precursor gate electrodes 88P (68P) and 168P (148P) respectivelyfor IGFETs 42 (40) and 122 (120). Portions of the composite gatedielectric underlying precursor gate electrodes 88P (68P) and 168P(148P) respectively constitute gate dielectric layers 86 (66) and 166(146) for IGFETs 42 (40) and 122 (120). Surface-adjoining parts of upperbody-material portions 54P (52P) and 134P (132P) situated generallybelow, and extending laterally slightly beyond the opposite endslength-wise of precursor gate electrodes 88P (68P) and 168P (148P),respectively constitute intended locations for channel zones 84 (64) and164 (144) of IGFETs 42 (40) and 122 (120).

A dielectric layer 208 of silicon oxide is thermally grown along theexposed surface of precursor polysilicon gate electrodes 88P (68P) and168P (148P) to seal them. Seal-oxide layer 208 is typically 8 nm inthickness. During the polysilicon sealing step, the thickness of thegate dielectric material located along the portions of the uppersemiconductor surface not covered by gate electrodes 88P (68P) and 168P(148P) increases slightly. This composite surface dielectric layer isindicated by reference symbol 210 in FIG. 16e.

A photoresist mask 212 is formed on the portions of dielectric layers208 and 210 overlying active region 48 (46) as shown in FIG. 16f. A pairof laterally separated heavily doped p-type precursor source/drainextensions 160EP and 162EP (140EP and 142EP) for p-channel IGFET 122(120) are defined in n upper body-material portion 134P (132P) along theupper semiconductor surface by ion implanting a species of a p-typedopant at a heavy dosage through uncovered portions of surfacedielectric layer 210 and into a pair of laterally separatedsurface-adjoining segments of body-material portion 134P (132P). Thematerial consisting of photoresist mask 212, field insulation 44, gateelectrode 168P (148P), and the portion of seal oxide 208 situated alongelectrode 168P (164P) forms a shield that largely blocks the p-typesource/drain extension dopant from simultaneously passing through theupper semiconductor surface section directly underlying the shield.

The dosage of the p-type source/drain extension dopant is 3×10¹³—1×10¹⁴ions/cm², typically 6×10¹³ ions/cm². The p-type source/drain extensiondopant may consist of boron in elemental form or in the form of borondifluoride. For the typical case in which the p-type source/drainextension dopant consists of boron in the form of boron difluoride, theimplantation energy is 8-15 KeV, typically 10 KeV.

With photoresist mask 212 in place, a halo doping operation is performedfor p-channel IGFET 122 (120) by appropriately introducing an n-typehalo dopant into upper body-material portion 134P (132P) at a heavydosage. For short-channel IGFET 122, the halo doping is performed insuch a manner as to define a pair of heavily doped n-type precursor haloportions that merge together to establish a single heavily doped n-typecomposite precursor halo pocket portion 184P in the body-materialportion intended for channel zone 164. See FIG. 16g. The merger of thetwo n+ precursor halo portions to define n+ precursor halo pocketportion 184P for IGFET 122 may occur during the halo doping operation asindicated in the example of FIG. 16g or later in the fabricationprocess. N+ halo pocket 184P extends laterally fully across the intendedlocation for channel zone 164 and at least partway across (the locationfor) each of source/drain extensions 160EP and 162EP. FIG. 16g depictsan example in which halo pocket 184P extends fully across (the locationsfor) extensions 160EP and 162EP and thus fully across active region 128.

For long-channel IGFET 120, the halo doping causes a pair of laterallyseparated heavily doped n-type precursor halo pocket portions 180P and182P (not shown) to be defined in the body-material portion intended forchannel zone 144. N+ halo pocket portion 180P extends laterally partwayacross the intended location for channel zone 144 and at least partwayacross (the location for) source/drain extension 140EP. Similarly, n+halo pocket portion 182P extends laterally partway across the intendedlocation for channel zone 144 and at least partway across (the locationfor) source/drain extension 142EP.

Shadowing by items such as photoresist mask 212 and adjacent gateelectrodes, including any material situated along the gate electrodes,may cause halo pocket 184P (180P and 182P) not to extend fully across(the locations for) source/drain extensions 160EP and 162EP (140EP and142EP). See the models discussed below in connection with FIGS. 34, 35,and 36. In the most extreme case, halo pocket 184P (180P and 182P)extends only slightly across (the location for) extensions 160EP and162EP (140EP and 142EP). When such shadowing occurs, electricallyinconsequential portions of the n-type halo dopant may enter (thelocations for) extensions 160EP and 162EP (140EP and 142EP) at locations(not indicated) spaced laterally apart from halo pocket 184P (180P and182P).

The n-type halo dopant for IGFET 122 (120) is introduced by ionimplantation through uncovered portions of surface dielectric layer 210and through a pair of laterally separated primary sections of the uppersemiconductor surface. The material consisting of photoresist mask 212,field insulation 44, gate electrode 168P (148P), and the portion of sealoxide 208 situated along electrode 168P (148P) forms a shield whichlargely blocks the n-type halo dopant from simultaneously passingthrough the upper semiconductor surface section directly underlying theshield.

Portions of the n-type halo dopant penetrate laterally under precursorgate electrode 168P (148P) from generally opposite directions along thelength of IGFET 122 (120), i.e., laterally inward from the oppositetransverse sides of gate electrode 168P (148P). In particular, then-type halo dopant is of such characteristics and is implanted intoupper body-material portion 134P (132P) in such a way and subsequentprocessing is performed on the semiconductor structure in such a mannerthat, in the final semiconductor structure, the n-type halo dopant haspenetrated considerably further laterally under gate electrode 168 (148)from both directions along the IGFET's length than has the p-typesource/drain extension dopant.

The concentration of the n-type halo dopant reaches a local surfaceminimum along the upper semiconductor surface in the intended locationfor channel zones 164 (144) of IGFET 122 (120) at a position between theintended locations for source/drain zones 160 and 162 (140 and 142) andincreases in moving away from the location of the local surface minimumtowards the intended location for each of source/drain zones 160 and 162(140 and 142). Since n⁺ precursor extended halo pocket 184P forshort-channel IGFET 122 is defined by the merger of two n⁺ halo portionsthat penetrate below precursor gate electrode 168P, the local surfaceminimum in the concentration of the halo dopant in the intendedchannel-zone location for IGFET 122 is greater than zero and occursapproximately at the location of the channel center. The location of thelocal surface minimum in the concentration of the halo dopant in theintended channel-zone location for IGFET 122 can be described asoccurring approximately at the center point of an imaginary line (notshown) extending between the intended locations for source/drain zones160 and 162 along the upper semiconductor surface.

The local surface minimum in the concentration of the n-type halo dopantin the intended location for channel zones 144 of long-channel IGFET 120is zero because n⁺ precursor halo pockets 180P and 182P do not merge.This zero value for the local surface minimum in the concentration ofthe halo dopant in the intended channel-zone location for IGFET 120occurs along a non-zero portion of an imaginary line (not shown)extending between the intended locations for source/drain zones 140 and142 along the upper semiconductor surface. The local surface minimum inthe concentration of the halo dopant is, of course, greater forshort-channel IGFET 122 than for long-channel IGFET 120. Also, theamount of n-type halo dopant in the intended channel-zone location forIGFET 122 is approximately the same as the amount of n-type halo dopantin the intended channel-zone

Angled ion implantation is preferably utilized to introduce the n-typehalo dopant into upper-body material portion 134P (132P). Ions of aspecies of the n-type halo dopant are implanted into the semiconductorbody at an average tilt angle α of at least 15°, normally at least 25°,relative to a direction perpendicular to the lower semiconductor surfaceand therefore generally perpendicular to the upper semiconductorsurface. In FIG. 16g, reference symbol 214 represents a vertical lineextending perpendicular to the lower semiconductor surface and thusgenerally perpendicular to the upper semiconductor surface. Tilt angle αis measured relative to a vertical line such as line 214.

Tilt angle α is also generally determined in a vertical plane, i.e., aplane extending generally perpendicular to the upper semiconductorsurface, extending approximately in the direction of the channel lengthof the IGFET being fabricated. Accordingly, there are two determinationsof angle α and thus two parts (or steps) in the n-type haloimplantation. As shown in FIG. 16g, angle α is measured both to theright of vertical line 214 to a principal impingement axis 216 and tothe left of line 214 to another principal impingement axis 218. Part,typically approximately one half, of the ions of the n-type halo dopantspecies impinge on the semiconductor body generally along, i.e.,approximately parallel on the average to, principal axis 216. Theremainder, likewise typically approximately one half, of the ions of then-type halo dopant species impinge on the semiconductive body generallyalong principal axis 218. To implement both determinations of angle α,either the semiconductor body can be rotated appropriately about avertical axis, or the ion implantation beam can be suitably moved.

Principal impingement axes 216 and 218 are typically largely fixedduring the implantation of the n-type halo dopant. Nonetheless,principal axes 216 and 218 can be varied during the n-type halo dopantimplantation. When axes 216 and 218 are fixed, the value of tilt angle αfor axis 216 can differ from the value of angle α for axis 218. The twodeterminations of angle α are usually close to each other, normallywithin 10° of each other, preferably within 5° of each other, andtypically approximately equal to each other.

The angled implantation arrangement illustrated in. FIG. 16g is aversion of a more general angled ion implantation arrangement in whichimpingement of ions of the n-type halo dopant species on photoresistmask 212 and the underlying semiconductor body is also characterized byan azimuthal (rotational) angle measured from some reference line in aplane extending generally parallel to the upper semiconductor surface.For instance, the reference line typically extends in the longitudinaldirection for IGFET 122 (120), i.e., along the channel length from thelocation for one of source/drain zones 160 and 162 (140 and 142) to thelocation for the other. Using that definition, FIG. 16g depicts thesituation in which the ions impinging on the semiconductor body at oneof the determinations of tilt angle α are at an average azimuthal angleof 0° while the ions impinging on the semiconductor body at the otherdetermination of angle a are at an average azimuthal angle of 180°.Hence, the angled implantation arrangement in FIG. 16g is performed fromtwo opposite azimuthal orientations, one being in the positive directionof the reference line extending in the longitudinal direction, and theother being in the negative direction of the reference line.

The angled implantation arrangement of FIG. 16g is typically preferredif no significant amount of undesired ion channeling occurs when ions ofthe n-type halo dopant species enter the semiconductor body from the twoazimuthal orientations of FIG. 16g. Should a significant amount ofundesired ion channeling occur with the azimuthal arrangement of FIG.16g, azimuthal orientations different from that represented in FIG. 16gare utilized to reduce the ion channeling.

In general, again using the preceding definition of azimuthal angle, onepart of the ions of the n-type halo dopant species impinge onphotoresist mask 212 and the semiconductor body at a first averageazimuthal angle which normally differs from 0° by no more than 60°,preferably differs from 0° by no more than 45°, and typically differsfrom 0° by 30-35° to safely avoid ion channeling. The remainder of theions impinge on mask 212 and the semiconductor body at a second averageazimuthal angle which normally differs from 180° by no more than 60°,preferably differs from 180° by no more than 45°, and typically differsfrom 180° by 30-35° to safely avoid ion channeling. The two azimuthalangles normally differ by approximately 180°. Hence, one of theazimuthal angles is typically 30-35° (or −30-−35°) while the otherazimuthal angle is typically 210-215° (or 145-150°).

Principal impingement axis 216 is at one of the azimuthal anglesrelative to the reference line. Principal impingement axis 218 is at theother azimuthal angle relative to the reference line. When the azimuthalangles differ by approximately 180°, principal axes 216 and 218 crosseach other at an axial angle approximately equal to the sum of (a) tiltangle α for axis 216 and (b) tilt angle α for axis 218.

In the typical situation where the first and second azimuthal anglesrespectively differ from 0° and 180°, shadowing by items such as fieldinsulation 44 typically produces some transverse asymmetry, i.e.,asymmetry in the direction of the channel width, in the n-type halodoping across halo pocket 184P (180P and 182P). The degree of transversehalo asymmetry typically increases with increasing difference betweenthe first azimuthal angle and 0° and/or with increasing differencebetween the second azimuthal angle and 180°.

The transverse halo asymmetry can be partially or wholly overcome byappropriately performing each above-mentioned part of the n-type haloimplantation in two parts (or steps). Specifically, part of the n-typehalo implantation at the first azimuthal angle is conducted at anazimuthal angle greater than 0° while the remainder of the implantationat the first azimuthal angle is conducted at an azimuthal angle lessthan 0°, both of these azimuthal angles meeting the broad specificationsgiven above for the first azimuthal angle. The two azimuthal angles may,for example, differ from 0° by approximately equal amounts and thus beapproximately equal in magnitude but of opposite sign.

Similarly part of the n-type halo implantation at the second azimuthalangle is conducted at an azimuthal angle greater than 180° while theremainder of the n-type halo implantation at the second azimuthal angleis conducted at an azimuthal angle less than 180°, both of theseazimuthal angles meeting the broad specifications given above for thesecond azimuthal angle. The two azimuthal angles used here may, forinstance, differ from 180° by approximately equal amounts. Furtherinformation on the azimuthal factors involved in angled ion implantationof the present type is presented in Wang et al, U.S. patent applicationSer. No. 09/540,734, filed 31 Mar. 2000, now allowed, the contents ofwhich are incorporated by reference to the extent not already disclosedherein.

The geometry of the semiconductor structure during the n-type halodopant implantation is arranged so that the n-type halo dopant speciesions directed along principal impingement axis 216 pass through part orall of the section of surface dielectric layer 210 to the immediate leftof gate dielectric layer 166 (146) and, at least in part, end up in apart of upper body-material portion 134P (132P) located below precursorgate electrode 168P (148P) generally near its left-hand transverse side.Similarly, the n-type halo dopant species ions directed along principalimpingement axis 218 pass through part or all of the section of upperdielectric layer 210 to the immediate right of gate dielectric 166 (146)and, at least in part, end up in a part of upper portion 134P (132P)located below gate electrode 168P (148P) generally near its right-handtransverse side.

The total dosage of the n-type halo dopant species is 8×10¹²-3×10¹³ions/cm², typically 1.6×10¹³ ions/cm². Half of the n-type halo dosage ispreferably furnished along each of principal impingement axes 216 and218. Tilt angle α is typically 30° for the n-type halo implantation. Then-type halo dopant may consist of arsenic or phosphorus. For the typicalcase in which arsenic constitutes the n-type halo dopant, theimplantation energy is 125-175 KeV, typically 150 KeV. Photoresist mask212 is removed after completing the n-type halo implantation.

A photoresist mask 220 is formed on the portion of dielectric layers 208and 210 overlying active region 128 (126) as shown in FIG. 16h. A pairof laterally separated heavily doped n-type precursor source/drainextensions 80EP and 82EP (60EP and 62EP) for n-channel IGFET 42 (40) aredefined in p− upper body-material portion 54P (52P) along the uppersemiconductor surface by ion implanting a species of an n-type dopant ata heavy dosage through uncovered portions of surface dielectric layer210 and into a pair of laterally separated suggest-adjoining segments ofbody-material portion 54P (52P). The material consisting of photoresistmask 220, field insulation 44, gate electrode 88P (68P), and the portionof seal oxide 208 situated along electrode 88P (86P) forms a shield thatlargely blocks the n-type source/drain extension dopant fromsimultaneously passing through the upper semiconductor surface sectiondirectly underlying the shield.

The dosage of the n-type source/drain extension dopant is 3×10¹⁴-1×10¹⁵ions/cm², typically 5×10¹⁴ ions/cm². The n-type source/drain extensiondopant may consist of arsenic or phosphorus. For the typical case inwhich arsenic constitutes the n-type source/drain extension dopant, theimplantation energy is 10-30 KeV, typically 20 KeV.

With photoresist mask 220 in place, a halo doping operation is performedfor n-channel IGFET 42 (40) by appropriately introducing theabove-mentioned p-type halo dopant into upper body-material portion 54P(52P) at a heavy dosage. For short-channel IGFET 42, the halo doping isperformed in such a way as to define a pair of heavily doped p-typeprecursor halo pocket portions that merge together to establish a singleheavily doped p-type composite precursor halo pocket portion 104P in thebody-material portion intended for channel zone 84. See FIG. 16i. Themerger of the two p+ precursor halo portions to define p+ precursorextended halo pocket portion 104P for IGFET 42 may occur during thep-type halo doping operation as indicated in the example of FIG. 16i orlater in the fabrication process. P+ halo pocket 104P extends laterallyacross the intended location for channel zone 84 and at least partwayacross (the location for) each of source/drain extensions 80EP and 82EP.FIG. 16i illustrates an example where halo pocket 104P extends fullyacross (the locations for) extensions 80EP and 82EP and therefore fullyacross active region 48.

For long-channel IGFET 40, the halo doping causes a pair of laterallyseparated heavily doped p-type precursor halo pocket portions 100P and102P (not shown) to be defined in the body-material portion intended forchannel zone 64. P+ halo pocket portion 100P extends laterally partwayacross the intended location for channel zone 84 and at least partwayacross (the location for) source/drain extension 60EP. Similarly, p+halo pocket 102P extends laterally partway across the intended locationfor channel zone 84 and at least partway across (the location for)source/drain extension 62EP.

As discussed below in connection with FIGS. 34-36, shadowing byphotoresist mask 220 and adjacent gate electrodes, including materialsituated along the electrodes, may cause halo pocket 104P (100P and102P) not to extend fully across (the locations for) source/drainextensions 80EP and 82EP (60EP and 62EP). At the extreme, halo pocket104P (100P and 102P) extends only slightly across (the locations for)extensions 80EP and 82EP (60EP and 62EP). When such shadowing is afactor, electrically inconsequential portions of the p-type halo dopantmay enter (the locations for) extensions 80EP and 82EP (60EP and 62EP)at locations (not indicated) laterally separated from halo pocket 104P(100P and 102P).

The p-type halo dopant for IGFET 42 (40) is introduced by implantingions of a species of the dopant through uncovered portions of surfacedielectric layer 210 and through a pair of laterally separated primarysections of the upper semiconductor surface. The material consisting ofphotoresist mask 220, field insulation 44, precursor gate electrode 88P(68P), and the portion of seal oxide 208 situated along electrode 88P(68P) forms a shield which largely blocks the p-type halo dopant fromsimultaneously passing through the upper semiconductor surface sectiondirectly underlying the shield.

Portions of the p-type halo dopant penetrate laterally under precursorgate electrode 88P (68P) from generally opposite directions along thelength of IGFET 42 (40), i.e., laterally inward from the oppositetransverse sides of gate electrode 88P (68P). Specifically, the p-typehalo dopant is of such characteristics and is implanted into upperbody-material portion 54P (52P) in such a manner and subsequentprocessing is performed on the semiconductor structure in such a waythat, in the final semiconductor structure, the p-type halo dopant haspenetrated considerably further laterally under gate electrode 88 (68)from both directions along the IGFET's length than has the n-typesource/drain extension dopant.

The concentration of the p-type halo dopant reaches a local surfaceminimum along the upper semiconductor surface in the intended locationfor channel zone 84 (64) of IGFET 42 (40) at a position between theintended locations for source/drain zones 80 and 82 (60 and 62) andincreases in moving away from the location of the local surface minimumtowards the intended location for each of source/drain zones 80 and 82(60 and 62). Since p+ precursor extended halo pocket 104P forshort-channel IGFET 42 is defined by the merger of two p-type haloportions that penetrate below precursor gate electrode 88P, the localsurface minimum in the concentration of the p-type halo dopant in theintended channel-zone location for IGFET 42 is greater than zero andoccurs approximately at the location of the channel center. The locationof the local surface minimum in the concentration of the halo dopant inthe intended channel-zone location for IGFET 42 can be described asoccurring approximately at the center point of an imaginary line (notshown) extending between the intended locations for source/drain zones80 and 82 along the upper semiconductor surface.

The local surface minimum in the concentration of the p-type halo dopantin the intended location for channel zone 64 of long-channel IGFET 40 iszero because p+ precursor halo pockets loop and 102P do not merge. Thiszero value for the local surface minimum in the concentration of thehalo dopant in the intended channel-zone location for IGFET 40 occursalong a non-zero portion of an imaginary line (not shown) extendingbetween the intended locations for source/drain zones 60 and 62 alongthe upper semiconductor surface. The local surface minimum in theconcentration of the halo dopant is, or course, greater forshort-channel IGFET 42 than for long-channel IGFET 40. Also, the amountof p-type halo dopant in the intended channel-zone location for IGFET 42is approximately the same as the amount of p-type halo dopant in theintended channel-zone location for IGFET 40.

Angled ion implantation is preferably employed to introduce the p-typehalo dopant into upper body-material portion 54P (52P). The angledimplantation for the p-type halo dopant is performed in the mannergenerally described above for the angled implantation of the n-type halodopant. Accordingly, average tilt angle α is at least 15°, normally atleast 25°, for implanting the p-type halo dopant. FIG. 16i repeatsvertical line 214 and principal impingement axes 216 and 218 for the twodeterminations of angle α. The azimuthal considerations described abovefor the n-type halo implantation likewise apply to the p-type haloimplantation.

The geometry of the semiconductor structure during the p-type halodopant implantation is arranged so that the p-type halo dopant speciesions directed along principal impingement axis 216 pass through part orall of the section of upper dielectric layer 210 to the immediate leftof gate dielectric layer 86 (66) and, at least in part, end up in thepart of upper body-material portion 54P (52P) located below precursorgate electrode 88P (68P) generally near its left-hand transverse side.Similarly, the p-type halo dopant species ions directed along principalimpingement axis 218 pass through part or all of the section ofdielectric layer 210 to the immediate right of gate dielectric layer 86(66) and, at least in part, end up in part of upper portion 54P (52P)located below gate electrode 88P (68P) generally near its right-handtransverse side.

The total dosage of the p-type halo dopant species is 8×10¹²-1.5×10¹³ions/cm², typically 1×10¹³ ions/cm². Half of the p-type halo dosage ispreferably furnished along each of impingement axes 216 and 218. Tiltangle a is typically 35° for the p-type halo implantation. The p-typehalo dopant may consist of boron in elemental form or in the form ofboron difluoride. For the typical case in which the p-type halo dopantconsists of boron in the form of boron difluoride, the implantationenergy is 50-100 KeV, typically 75 KeV. Photoresist mask 220 is removedafter completing the p-type halo implantation.

The n-type halo doping operation can be performed before the p-typesource/drain extension doping operation. Photoresist mask 212 is thencreated before the n-type halo doping and removed after the p-typesource/drain extension doping. Similarly, the p-type halo dopingoperation can be performed before the n-type source/drain extensiondoping operation. In this case, photoresist mask 220 is created beforethe p-type halo doping and removed after the n-type source/drainextension doping. In addition, the combination of the n-typesource/drain extension doping, the p-type halo doping, and theformation/removal of photoresist 220 can be performed before thecombination of the p-type source/drain extension doping, the n-type halodoping, and the formation/removal of photoresist 212.

An extended-time furnace anneal, referred to as the source/drainextension anneal, at a relatively low temperature may be performed atthis point to remove defects caused by the heavy dosages of thesource/drain extension implants. When performed, the source/drainextension anneal is done at 540-560° C., typically 550° C., for 60-120min., typically 90 min., in a non-reactive environment or in a reactiveoxygen-containing environment. When the source/drain extension anneal isdone in a reactive oxygen-containing environment, the thicknesses ofdielectric layers 208 and 210 increases slightly.

Next, dielectric spacer pair 90 and 92 (70 and 72) for IGFET 42 (40) anddielectric spacer pair 170 and 172 (150 and 152) for IGFET 122 (120) areprovided respectively along the transverse sidewalls of precursor gateelectrodes 88P (68P) and 168P (148P). See FIG. 16j. In particular, athin layer of tetraethyl orthosilicate (again, “TEOS”) is deposited ontop of the structure, typically to a thickness of 30 nm. A considerablythicker layer of silicon nitride is deposited on the TEOS layer,typically to a thickness of 100 nm. An anisotropic etch is performedgenerally perpendicular to the upper semiconductor surface to removelargely all the silicon nitride, except for portions situated on theTEOS layer to the sides of gate electrodes 88P (68P) and 168P (148P).Each gate sidewall spacer 90, 92, 170, or 172 (70, 72, 150, or 152)thereby consists of a main silicon nitride portion situated on a TEOSfooter which overlies portions of dielectric layers 208 and 210.

The portions of the TEOS layer not covered by the silicon nitrideportions of gate sidewall spacers 90, 92, 170, and 172 (70, 72, 150, and152) are removed. The underlying portions of dielectric layers 208 and210 are partially, but not totally, removed. Items 222 and 224 in FIG.16j respectively indicate the remainders of dielectric layers 210 and208 not covered by sidewall spacers 90, 92, 170, and 172 (70, 72, 150and 152).

A photoresist mask 226 is formed on sidewall spacers 170 and 172 (150and 152) and on the portions of dielectric layers 222 and 224 overlyingactive region 128 (126) as shown in FIG. 16k. N++ main source/drainportions 80M and 82M (60M and 62M) for n-channel IGFET 42 (40) aredefined in upper body-material portion 54P (52P) by ion implanting aspecies of n-type dopant at a very heavy dosage through the uncoveredportions of dielectric layer 222 and into a pair of laterally separatedsurface-adjoining segments of body-material portion 54P (52P).

When n-type source/drain zones 80 and 82 (60 and 62) for IGFET 42 (40)are to be provided with respective lower portions (not shown) morelightly doped than n++ main source/drain portions 80M and 82M (60M and62M), the more lightly doped lower source/drain portions are defined inupper body-material portion 54P (52P) by ion implanting a species of ann-type dopant at a heavy dosage through the uncovered portions ofdielectric layer 222, through the same two upper semiconductor surfacesections through which the n-type main source/drain dopant species isimplanted, and into a pair of laterally separated segments ofbody-material portion 54P (52P). The material consisting of photoresistmask 226, field insulation 44, gate electrode 88P (68P), and spacers 90and 92 (70 and 72) forms a shield that largely blocks the n-type dopantsfor main source/drain portions 80M and 82M (60M and 62M) and the lowersource/drain portions from simultaneously passing through the uppersemiconductor surface section directly underlying the shield.

The n-type dopant which defines the lower source/drain portions isfurnished at a considerably lighter dosage, but penetrates significantlydeeper (on the average) below the upper semiconductor surface, i.e., hasa longer range, than the n-type dopant which defines main source/drainportions 80M and 82M (60M and 62M). As a result, the dopant profilealong a vertical cross-section through the center lengthwise of each ofmain source/drain portions 80M and 82M (60M and 62M) has a gradedcharacteristic in the vicinity of the pn junction between body-materialportion 54P (52P) and each of source/drain zones 80 and 82 (60 and 62).This graded characteristic, especially for source/drain zone 80 or 82(60 or 62) acting as the drain during IGFET operation, reduces thejunction capacitance, thereby increasing the switching speed of IGFET 42(40).

Either the implantation for main source/drain portions 80M and 82M (60Mand 62M) or the implantation for the more lightly doped lower portionsof source/drain zones 80 and 82 (60 and 62) may be done first. In atypical implementation, the lighter-dosage/longer-range n-typeimplantation for the lower source/drain portions is performed first at adosage of 2×10¹³-4×10¹³ ions/cm², typically 3.5×10¹³ ions/cm². Althoughthe lower portions of source/drain zones 80 and 82 (60 and 62) are morelightly doped that main source/drain portions 80M and 82M (60M and 62M),the lower source/drain portions are heavily doped n-type. The n-typelower source/drain dopant may consist of phosphorus or arsenic. For thetypical case in which phosphorus constitutes the n-type lowersource/drain dopant, the implantation energy is 50-70 KeV, typically 60KeV.

The heavier-dosage/shorter range n-type implantation for mainsource/drain portions 80M and 82M (60M and 62M) is subsequently done ata dosage of 5×10¹⁵-1×10¹⁶ ions/cm², typically 7×10¹⁵ ions/cm². Then-type main source/drain may consist of arsenic or phosphorus. For thetypical case in which arsenic constitutes the n-type main source/draindopant, the implantation energy is 40-60 KeV, typically 50 KeV.

In the final graded-junction structure for n-channel IGFET 42 (40), n++main source/drain portions 80M and 82M (60M and 62M) are formed with (a)the two parts of the heavier-dosage/shorter-range n-type mainsource/drain dopant situated respectively in the locations for mainportions 80M and 82M (60M and 62M) and (b) two considerably smallerparts of the lighter-dosage/longer-range n-type lower source/draindopant situated respectively in the n++ main source/drain locations.Since main source/drain portions 80M and 82M (60M and 62M) containconsiderably more of the heavier-dosage/shorter-range n-type dopant thanof the lighter-dosage/longer-range n-type dopant, main portions 80M and82M (60M and 62M) are largely formed by the heavier-dosage/shorter-rangen-type main source/drain dopant. The n+ lower source/drain portions areformed with the parts of the lighter-dosage/shorter-range n-type lowersource/drain dopant located respectively below main portions 80M and 82M(60M and 62M). The combination of each main source/drain portion 80M or82M (60M or 62M) and the underlying lower source/drain portion forms acomposite main source/drain portion having an overall very heavy dopingn-type.

Due to the presence of spacers 90 and 92 (70 and 72), main source/drainportions 80M and 82M (60M and 62M) respectively partially overlapprecursor source/drain extensions 80EP and 82EP (60EP and 62EP). Mainportions 80M and 82M (60M and 62M) are doped much heavier than, andextend deeper into upper body-material portion 54P (52P) than, precursorextensions 80EP and 82EP (60EP and 62EP). Hence, the remainders ofprecursor extensions 80EP and 82EP (60EP and 62EP) now respectivelyconstitute n+ source/drain extensions 80E and 82B (60E and 62E).Similarly, the p-type remainders of precursor body material 50P,precursor upper body-material portion 54P (52P), and precursor extendedhalo pocket 104P (separate precursor halo pockets 100P and 102P) nowrespectively constitute p-type body material 50, p-type upperbody-material portion 54 (52), and p+ extended halo pocket 104 (separatep+ halo pockets 100 and 102).

Some p-type and n-type dopant was introduced into precursor gateelectrode 88P (68P) during the p-type and n-type doping operationsperformed prior to the n-type main and lower-portion source/drain dopingoperations. However, the total amount of p-type dopant introduced intoprecursor gate electrode 88P (68P) is insignificant compared to theamount of n-type dopant introduced into gate electrode 88P (68P) duringthe n-type main source/drain doping. Accordingly, precursor gateelectrode 88P (68P) is now n++ gate electrode 88 (68). Also, gateelectrode 88 (68) is of the same conductivity type, i.e., n-type, assource/drain zones 80 and 82 (60 and 62).

After removing photoresist mask 226, a photoresist mask 228 is formed onsidewall spacers 90 and 92 (70 and 72) and on the portion of dielectriclayers 222 and 224 overlying active region 48 (46). See FIG. 16l. P++main source/drain portions 160M and 162M (140M and 142M) for p-channelIGFET 122 (120) are defined in upper body-material portion 134P (132P)by ion implanting a species of a p-type dopant at a very heavy dosagethrough the uncovered portions of dielectric layer 222 and into a pairof laterally separated surface-adjoining segments of body-materialportion 134P (132P).

When source/drain zones 160 and 162 (140 and 142) of IGFET 122 (120) areto be provided with respective lower portions more lightly doped thanp++ main source/drain portions 160M and 162M (140M and 142M), the morelightly doped lower source/drain portions are defined in upperbody-material portion 134P (132P) by ion implanting a species of ap-type dopant at a heavy dosage through the uncovered portions ofdielectric layer 222 and through the same two upper semiconductorsurface sections through which the p-type main source/drain dopantspecies is implanted. Although the more lightly doped lower portions ofsource/drain zones 160 and 162 (140 and 142) are not shown in FIG. 16l,FIGS. 18a and 18 b respectively illustrate the final structures of longp-channel IGFET 120 and short p-channel IGFET 122 when they have morelightly doped lower source/drain portions. Items 140L and 142L in FIG.18a are the more lightly doped lower portions of source/drain zones 140and 142 in long-channel IGFET 120. Items 160L and 162L in FIG. 18b arethe more lightly doped lower portions of source/drain zones 160 and 162in short-channel IGFET 122. The material formed with photoresist mask228, field insulation 44, precursor gate electrode 168P (148P), andspacers 170 and 172 (150 and 152) forms a shield that largely blocks thep-type source/drain dopants for main source/drain portions 160M and 162M(140M and 142M) and lower source/drain portions 160L and 162L (140L and142L) from simultaneously passing through the upper semiconductorsurface section directly underlying the shield.

The p-type dopant which defines lower source/drain portions 160L and162L (140L and 142L) is furnished at a considerably lighter dosage, butpenetrates significantly deeper (on the average) below the uppersemiconductor surface, i.e., has a longer range, than the p-type dopantwhich defines main source/drain portions 160M and 162M (140M and 142M).Consequently, the dopant profile along a vertical cross-section throughthe center lengthwise of each of main source/drain portions 160M and162M (140M and 142M) has a graded characteristic in the vicinity of thepn junction between body-material portion 134P (132P) and each ofsource/drain zones 160 and 162 (140 and 142). This gradedcharacteristic, especially for source/drain zone 160 or 162 (140 or 142)acting as the drain during IGFET operation, reduces the junctioncapacitance so as to increase the switching speed of IGFET 122 (120).

Either the implantation for main source/drain portions 160M and 162M(140M and 142M) or the implantation for lower source/drain portions 160Land 162L (140L and 162L) may be done first. In a typical implementation,the lighter-dosage/longer-range p-type implantation for lowersource/drain portions 160L and 162L (140L and 142L) is performed firstat a dosage of 3×10¹³-6×10¹³ ions/cm², typically 4×10¹³ ions/cm².Although lower source/drain portions 160L and 162L (140L and 142L) aremore lightly doped than main source/drain portions 160M and 162M (140Mand 142M), lower source/drain portions 160L and 162L (140L and 142L) areheavily doped p-type. The p-type lower source/drain dopant may consistof boron in elemental form or in the form of boron difluoride. For thetypical case in which the p-type lower source/drain dopant consists ofelemental boron, the implantation energy is 20-35 KeV, typically 25 KeV.

The heavier-dosage/shorter-range p-type implantation for mainsource/drain portions 160M and 162M (140M and 142M) is subsequently doneat a dosage of 2.5×10¹⁵-1×10¹⁶ ions/cm², typically 5×10¹⁵ ions/cm². Thep-type main source/drain dopant may consist of boron in the form ofboron difluoride or in elemental form. For the typical case in which thep-type main source/drain dopant consist of boron in the form of borondifluoride, the implantation energy is 20-30 KeV, typically 25 KeV.

In the final graded-junction structure for IGFET 122 (120), p++ mainsource/drain portions 160M and 162M (140M and 142M) are formed with (a)the two parts of the heavier-dosage/shorter-range n-type mainsource/drain dopant situated respectively in the locations for mainportions 160M and 162M (140M and 142M) and (b) two considerably smallerparts of the lighter-dosage/longer-range p-type lower source/draindopant situated respectively in the p++ main source/drain locations.Inasmuch as main source/drain portions 160M and 162M (140M and 142M)contain considerably more of the heavier-dosage/shorter-range p-typedopant than of the lighter-dosage/longer-range p-type dopant, mainportions 160M and 162M (140M and 142M) are largely formed by theheavier-dosage/shorter-range p-type dopant. P+ lower source/drainportions 160L and 162L (140L and 142L) are formed with the parts of thelighter-dosage/longer-range p-type lower source/drain dopant locatedrespectively below main source/drain portions 160M and 162M (140M and142M). The combination of each main source/drain portion 160M or 162M(14CM or 142M) and underlying lower source/drain portion 160L or 162L(140L or 142L) forms a composite main source/drain portion having anoverall very heavy doping p-type.

As indicated above, the species of p-type dopant utilized in definingmain source/drain portions 160M and 162M (140M and 142M) is typicallyboron difluoride whereas the species of p-type dopant employed indefining lower source/drain portions 160L and 162L (140L and 142L) istypically elemental boron. Boron difluoride has a greater molecularweight than boron. Consequently, the species of p-type dopant utilizedin the heavier-dosage/shorter-range implantation for main source/drainportions 160M and 162M (140M and 142M) is of greater molecular weightthan the species of the p-type dopant utilized in thelighter-dosage/longer-range implantation for lower source/drain portions160L and 162L (140L and 142L).

The energy needed to ion implant a species of semiconductor dopant to agiven depth into a semiconductor body generally increases as themolecular weight of the dopant species increases. With conventionalcommercially available state-of-the-art ion implantation equipment, itis commonly difficult to accurately control ion implantation atimplantation energy below 10 KeV. Boron is an atom of relatively lowmolecular weight. The implantation energy that would be needed toimplant ions of elemental boron to define main source/drain portions160M and 162M (140M and 142M) is less than 10 KeV, e.g., 5 KeV. Byperforming the heavier-dosage/shorter-range implantation for mainsource/drain portions 160M and 162M (140M and 142M) with a p-type dopantspecies of greater molecular weight than the p-type dopant utilized inthe lighter-dosage/longer-range implantation for lower source/drainportions 160L and 162L (140L and 142L), the implantation energy of theheavier-dosage/shorter-range implantation for main portions 160M and162M (140M and 142M) is increased. This facilitates accurately formingthe graded-junction characteristics for source/drain zones 160 and 162(140 and 142).

FIG. 18c presents an example of how net dopant concentration N_(N)varies in the vertical direction through source/drain zone 160 or 162(140 or 142) of p-channel IGFET 122 (120) along a cross-sectionextending vertically through the center length-wise of main source/drainportion 160M or 162M (140M or 142M). Vertical distance (or depth) y inFIG. 18c is measured from the upper semiconductor surface downward.Curve segments 160M*, 162M*, 160L*, 162L*, and 134* (140M*, 142M*,140L*, 142L*, and 132*) respectively represent the net dopantconcentrations in components 160M, 162M, 160L, 162L, and 134 (140M,142M, 140L, 142L and 132). As shown by curve segments 160M* and 160L*(140M* and 140L*) or 162M* and 162L* (or 142M* and 142L*), the verticaldopant profile for main portion 160M (140M) or 162M (142M) merges intothe vertical dopant profile for lower portion 160L (140L) or 162L(142L).

The dashed line in FIG. 18c indicates how the vertical dopant profilesfor main source/drain portions 160M and 162M (140M and 142M) wouldappear if more lightly doped lower source/drain portions 160L and 162L(140L and 142L) were absent, i.e., IGFET 122 (120) did not have gradedjunction characteristics. Comparison of the composite vertical dopantprofile formed by curve segments 160M* and 160L* (140M* and 140L*) or162M* and 162L* (or 140L* and 142L*) to the non-graded vertical dopantprofile represented by the combination of the dashed-curve segment andcurve segment 160M* or 162M (140M* or 142M*) shows that, in moving fromthe pn junction between body-material portion 134 (132) and source/drainzones 160 or 162 (140 or 142), the vertical dopant profile through thecenter length-wise of main source/drain portion 160M or 162M (140M or142M) rises less sharply, on the average, to the maximum net dopantconcentration in main portion 160M or 162M (140M or 142M) than whatwould occur if the p-type dopant which defines zones 160 or 162 (140 or142) along that cross-section were ion implanted at a single energy,i.e., lower source/drain portion 160L or 162L (140L or 142L) wereabsent. Hence, IGFET 122 (120) has a graded junction characteristic.

Due to the presence of spacers 170 and 172 (150 and 152), mainsource/drain portions 160M and 162M (140M and 142M) respectivelypartially overlap precursor source/drain extensions 160EP and 162EP(140EP and 142EP). Main portions 160M and 162M (140M and 142M) are dopedmuch heavier than, and extend deeper into upper body-material portion134P (132P) than, precursor extensions 160EP and 162EP (140EP and142EP). Hence, the remainders of precursors extensions 160EP and 162EP(140EP and 142EP) now respectively constitute p+ source/drain extensions160E and 162E (140E and 142E). Similarly, the n-type remainders ofprecursor upper body-material portion 134P (132P) and precursor extendedhalo pocket 184P (separate precursor halo pockets 180P and 182P) nowrespectively constitute n-type upper body-material portion 134 (132) andn+ extended halo pocket 184 (separate n+ halo pockets 180 and 182).

Some n-type and p-type dopant was introduced into precursor gateelectrode 168P (148P) during the n-type and p-type doping operationsprior to the p-type main and lower-portion source/drain dopingoperations. However, the total amount of n-type dopant introduced intoprecursor gate electrode 168P (148P) is insignificant compared to theamount of p-type dopant introduced into gate electrode 168P (148P)during the p-type main source/drain doping. Accordingly, precursor gateelectrode 168P (148P) is now p++ gate electrode 168 (148). Also, gateelectrode 168 (148) is of the same conductivity type (p-type) assource/drain zones 160 and 162 (140 and 142).

After removing photoresist mask 228, a capping layer (not shown) ofdielectric material is formed on top of the structure. The cappingdielectric layer typically consists of TEOS deposited by plasma-enhancedchemical vapor deposition to a thickness of 20 nm.

The semiconductor structure is now thermally annealed to repair latticedamage and activate the implanted source/drain and halo dopants. Theanneal, referred to as the source/drain-halo anneal, is of such a naturethat, at the end of the anneal, gate electrode 88 (68) extends slightlyover source/drain extensions 80E and 82E (160E and 162E), and gateelectrode 168 (148) extends slightly over source/drain extensions 160Eand 162E (140E and 142E).

The source/drain-halo anneal is typically an RTA at 1000-1100°,typically 1075° C., for 5-20 sec., preferably 10-15 sec., typically 12sec. The RTA typically includes a ramp-up from 525° C. to the annealtemperature at 60-90° C./sec., typically 75° C./sec. The RTA alsotypically includes a ramp-down from the anneal temperature to 700° C. at−40-−60° C./sec., typically −50° C./sec. The RTA is preferably done inan non-reactive environment, typically nitrogen. Alternatively, the RTAcan be done in a reactive oxygen-containing environment. In this case,silicon oxide grows along the upper silicon (both monosilicon andpolysilicon) surfaces. The so-grown oxide can replace the cappingdielectric layer.

The source/drain-halo anneal may include an extended-time furnace annealperformed before the source/drain-halo RTA at temperature considerablybelow the RTA annealing temperature. When performed, the furnace annealis done at 540-560° C., typically 550° C., for 60-120 min., typically 90min., in a non-reactive environment, or in a reactive oxygen-containingenvironment. The source/drain anneal is typically performed in anenvironment containing nitrogen and oxygen. When so done, thethicknesses of the dielectric layers along the upper silicon surfacesincrease slightly. Although the above-mentioned halo and source/drainimplantation steps define source/drain zones 80 and 82 (60 and 62),source/drain zones 160 and 162 (140 and 142), extended halo pocket 104(separate halo pockets 100 and 102), and extended halo pocket 184(separate halo pockets 180 and 182), the source/drain-halo annealconsisting of the RTA and the optional furnace anneal completes theformation of these regions.

The thin layers of dielectric material, including dielectric layers 222and 224, are removed along the upper semiconductor surface and along thetop surfaces of gate electrodes 88 (68) and 168 (148). Field insulation48 and spacers 90 and 92 (70 and 72) and 170 and 172 (150 and 142)remain substantially in place.

Metal silicide layers 94, 96, 98, 174, 176, and 178 (74, 76, 78, 154,156, and 168) are respectively formed along the upper surfaces ofcomponents 88, 80M, 82M, 168, 160M and 162M (68, 60M, 62M, 148, 140M,and 142M) as shown in FIG. 16m. This typically entails depositing a thinlayer of a suitable metal, typically cobalt, on the upper surface of thestructure and then causing the metal to react with the underlyingsilicon to form silicide layers 94, 96, 98, 174, 176, and 178 (74, 76,78, 154, 156, and 158). The unreacted metal is subsequently removed. Themetal silicide thickness is 25-35 nm, typically 30 nm. The metalsilicide formation completes the basic fabrication of n-channel IGFET 42(40) and p-channel IGFET 122 (120).

In subsequent process operations (not illustrated in the drawings), alayer of dielectric material is deposited on the upper surface of thestructure of FIG. 16m. Using a suitable photoresist mask, contactopenings are etched through the dielectric material down to metalsilicide layers 94, 96, 98, 174, 176, and 178 (74, 76, 78, 154, 156, and158). Alternatively, a self-aligned contact technique can be utilized toform contact openings through the dielectric material provided over thestructure.

A layer of interconnect metal typically consisting principally ofaluminum is deposited on the upper. surface of the resulting structure.Using a suitable photoresist mask, the interconnect metal is patternedinto a desired shape. The metallization process may include theformation of tungsten plugs. As desired, one or more further patternedmetal interconnect layers can be provided over the structure.

Experimental Program

Symmetrical n-channel and p-channel IGFETs were fabricated fromsemiconductor wafers to experimentally examine various aspects of theinvention. The numerical values for the process and device parametersutilized in manufacturing these IGFETs were based on process and devicecomputer simulations performed with the DIOS-ISE (process) andDESSIS-ISE (device) simulators using their default process and materialcoefficients as appropriate. See (a) Strecker, DIOS-ISE, ISE TCADManuals, Release 5, Vol. 3, Integrated Systems Engineering, 1997, and(b) Escoffier et al, DESSIS-ISE, ISE TCAD Manuals, Release 5, Vol. 5,Integrated Systems Engineering, 1997. Although all of the wafersutilized during the experimental program were processed to create bothn-channel and p-channel IGFETs, the experimental discussion presentedbelow deals primarily with n-channel IGFETs.

As used below in connection with an IGFET, the term “drawn channellength” means the value of the length of the IGFET's gate electrode asdrawn on the mask used to define the gate electrode. The drawn channellength, represented by symbol L_(DR), invariably differs from both theactual gate length and actual channel length L. However, drawn channellength L_(DR) is a very useful experimental parameter because actualchannel length L is generally difficult to measure precisely.

Halo and APT implants were applied in various ways to four lots ofcomplementary-IGFET wafers processed without graded junctioncharacteristics during the experimental program. These four lots ofwafers are referred to here as the first, second, third, and fourthlots. The wafers in each lot were processed the same and largelysimultaneously except for variations applied to splits (sub-lots) ofeach lot. Each split consisted of two or more wafers. Although thedifferent lots were not processed largely simultaneously, the sameprocessing steps were performed on all four wafer lots except for thevariations applied to the splits. Part of the information obtained fromthe four wafer lots was compared to information obtained from a fifthlot, referred to here as the reference lot, of complementary-IGFETwafers provided with APT implants and with halo implants but at very lowhalo dosages.

Each wafer was laid out so that the channel-length directions of then-channel IGFETs and the p-channel IGFETs whose parameters were measuredduring the experimental program extended parallel to one another. Thehalo implants, both n-type and p-type, were performed at azimuthalangles of 33°, 123°, 213°, and 303° to a reference line extending in thedirection of the channel lengths of these IGFETs. Each wafer alsoincluded n-channel and p-channel IGFETs whose channel-lengths directionsextended perpendicular to the channel-length directions of the IGFETswhose parameters were measured.

The graphical information generated for a particular wafer lot, or for asplit of that wafer lot, is generally indicated in the followingdrawings by the designation applied to that wafer lot, or the split ofthat wafer lot. Each of the data points on a curve described belownormally represents the average of multiple measurements. For example,each experimentally measured value of threshold voltage V_(T) at a givenvalue of drawn channel length L_(DR) for a particular V_(T)(L_(DR))curve is normally the average of multiple values of threshold voltageV_(T) at the given L_(DR) value.

One experiment entailed varying the dosage of the p-type ion-implantedhalo dopant in one direction as the dosage of the p-type ion-implantedAPT dopant was varied in the opposite direction for n-channel IGFETshaving a target threshold voltage of 0.4 V at a desired short-channeloperating value L_(DRC) of 0.18 μm for drawn channel length L_(DR). Foursplits, referred to here as process designs W, X, Y, and Z, of the firstlot of wafers were employed in this experiment. Using boron for thep-type halo and APT implants, designs X, Y, and Z had the followingp-type halo and APT dosages as determined by computer simulation:

a. Design X—high halo dose of 1.5×10¹³ ions/cm² and low APT dose of1.8×10¹² ions/cm²

b. Design Y—medium halo dose of 1.3×10¹³ ions/cm² and medium APT dose of3.8×10¹² ions/cm²

c. Design Z—low halo dose of 1.2×10¹³ ions/cm² and high APT dose of6.5×10¹² ions/cm²

Design W, which served as a baseline for comparison purposes, had thehigh p-type boron halo dose of design X but no p-type APT dopant.

The p-type boron halo implantations for the wafers in the first lot wereall performed at an implantation energy of 75 KeV and a tilt angle of40°. For inventive designs X, Y, and Z, the implantation energy for thep-type boron APT dopant was 50 KeV. The wafers in the first lot werelaid out so that the drawn value L_(DR) of channel length L varied from0.14 μm to 20 μm.

Subject to the variations applied to the different splits and subject tothe comments presented below, the wafers in the first lot were processedin the manner described above in connection with FIG. 16 but withoutgraded-junction characteristics. The gate dielectric layers in the firstwafer lot were grown according to the thermal schedule of FIG. 17. Thefinal anneal during the gate dielectric growth was performed withnitrous oxide so that the gate dielectric layers were all nitrided. Theannealing operations utilized in manufacturing the first lot were solelyRTAs. No isolation implants were utilized in processing the first lot.

FIG. 19a illustrates how threshold voltage V_(T) varied with drawnchannel length L_(DR) for n-channel IGFETs made according to processdesigns W, X, Y, and Z in the first wafer lot. The large circles on theV_(T)(L_(DR)) curves for designs W, X, Y, and Z present the V_(T) valuesmeasured at the desired short-channel L_(DRC) operating value of 0.18μm. The measured value of threshold voltage V_(T) at the L_(DR) value of0.18 μm is within 0.04 V of the target V_(T) value of 0.4 V for each ofinventive designs X, Y, and Z. This shows that the computer simulationprovided good approximation to the experimentally manufactured IGFETs.The measured V_(T) value at the L_(DRC) value of 0.18 μm for baselinedesign W is considerably below the target V_(T) value of 0.4 V becausethe p-type halo dose for design W was not adjusted sufficiently upwardto compensate for the absence of a p-type APT implant in design W.

As FIG. 19a indicates, short-channel threshold voltage roll-off isalleviated in the n-channel IGFETs fabricated according to all ofinventive designs X, Y, and Z. Among designs X, Y, and Z, the largestshift in threshold voltage roll-off toward lower threshold voltageoccurs with design X at the high halo dose. For the range of parametervalues considered in the experiment involving designs X, Y, and Z,increasing the halo dosage therefore leads to more alleviation inshort-channel threshold voltage roll-off.

Among inventive designs X, Y, and Z in the first wafer lot, design Y hadthe smallest variation in threshold voltage V_(T) in the immediatevicinity of the desired short-channel L_(DRC) value of 0.18 μm.Accordingly, design Y is largely optimum in terms of minimizing V_(T)variations that result from unintended manufacturing variations inchannel length L at the desired L_(DRC) value of 0.18 μm for shortn-channel IGFETs.

Examination of FIG. 19a indicates that short-channel threshold voltageroll-off is substantially alleviated in the n-channel IGFETs of baselinedesign W. However, the IGFETs of design W lack the APT implant and thusare more susceptible to punchthrough than the IGFETs of any of inventivedesigns X, Y, and Z. Also, in moving from the short-channel regime tothe long-channel regime, the IGFETs of design W have a lower value ofthreshold voltage V_(T) at any particular value of drawn channel lengthL_(DR) than the IGFETs of any of designs X, Y, and Z.

FIG. 19b depicts how extrapolated widthwise leakage current densityI_(DSSX) varied with widthwise drive current density I_(DSAT) forn-channel IGFETs of designs W, X, Y, and Z in the first wafer lot.Leakage current density I_(DSSX) is the extrapolated drain leakagecurrent per unit drain width determined by linearly extrapolating theactual drain current per unit drain width, in a logarithmic-linearrepresentation as a function of gate-to-source voltage V_(GS), to zerogate-to-source voltage. Drive current density I_(DSAT) is the draindrive current per unit drain width. Each point in FIG. 19b represents adifferent IGFET. Clusters of points are distinguishable generally to theleft in FIG. 19b, where each cluster is at a different target L_(DR)value.

As FIG. 19b indicates, leakage current density I_(DSSX) at anyparticular value of drive current density I_(DSAT) less thanapproximately 500 μA/μm decreased in going from design X to design Z. Inlight of the generally lower leakage current density, design Z may bepreferably from a practical viewpoint, especially in an IC having alarge number of long-channel IGFETS. Also, design Z yields an I_(DSAT)value of nearly 600 μA/μm at an actual leakage current density I_(DSS0),i.e., actual drain leakage current per unit drain width, of 1 nA/μm.This I_(DSAT) value is nearly 20% higher than that obtained withotherwise similar n-channel IGFETs which have generally flat channeldopant profiles along the upper semiconductor surface and thus whichlack the p-type halo implants used in the present invention.

FIG. 20 illustrates V_(T)(L_(DR)) variations for n-channel IGFETs ofdesign Z in the first lot of wafers compared to n-channel IGFETs of theabove-mentioned reference lot of wafers in which a p-type halo implantwas performed at a very low dosage, 5×10¹² ions/cm². A p-typethreshold-adjust implant was utilized for the n-channel IGFETs in thereference wafer lot. In contrast to the double-humped verticalchannel-center dopant profile, as illustrated generally in FIGS. 12a and12 b, of n-channel IGFETs configured according to the invention, thevertical channel-center dopant profile for the n-channel IGFEs in thereference lot was relatively flat. Rather than being performed solely byRTA as occurred in the first wafer lot, wafer annealing subsequent toion implantation of all the source/drain dopants in the reference lotwas performed in a furnace for extended time at temperature considerablybelow typical RTA temperature.

Computer simulation was utilized to provide the reference lot with atarget V_(T) value of 0.4 V at the desired short-channel L_(DRC)operating value of 0.18 μm. Aside from (a) performing the halo implantsat very low dosages, (b) the use of an extended-time furnace annealsubsequent to ion implantation of all the source/drain dopants, (c) theuse of threshold-adjust implants, and (d) variations in the parametervalues needed to provide the reference lot with the target 0.4-V valueof threshold voltage V_(T) at the L_(DRC) value of 0.18 μm, thereference lot was processed largely the same as inventive designs X, Y,and Z in the first lot.

The large dark circles on the V_(T)(L_(DR)) curves in FIG. 20 presentthe V_(T) values at the desired short-channel L_(DRC) operating value of0.18 μm. Comparison of the two V_(T)(L_(DR)) curves in FIG. 20 shows howshort-channel threshold voltage roll-off is greatly alleviated in thepresent invention. Short-channel threshold voltage roll-off was muchstronger (worse) with the reference wafer lot than with inventive designZ. In fact, aside from the fact that the reverse short-channel effect,i.e., the slow decrease in threshold voltage V_(T) with increasingchannel length in going from the short-channel regime to thelong-channel regime is not represented in the simplified conventionalV_(T)(L_(DR)) curve presented above in FIG. 1, the V_(T)(L_(DR)) curvefor the reference lot of FIG. 20 is quite similar to the V_(T)(L_(DR))curve of FIG. 1. This arises because, in both cases, the verticalchannel-center dopant profile is relatively flat.

A split, referred to as process design Y′, of the second lot of waferswas utilized to investigate the effect of incorporating nitrogen intothe gate dielectric material. As mentioned above, nitrogen wasincorporated into the gate dielectric layers of the first lot. Hence,the gate dielectric layers of the first lot, including design Y,consisted of silicon oxynitride. Design Y, of the second lot wasprocessed at the conditions of design Y of the first lot except that thefinal anneal during the gate dielectric growth in design y′ wasperformed with nitrogen rather than nitrous oxide. Accordingly, the gatedielectric layers in the IGFETs of design Y′ were non-nitrided, i.e.,the gate dielectric layers for design Y′ consisted substantially only ofsilicon oxide.

FIG. 21 depicts V_(T)(L_(DR)) variations for n-channel IGFETsmanufactured according to process designs Y and Y′, i.e., respectivelywith and without gate dielectric nitridation. As FIG. 21 shows, theV_(T)(L_(DR)) curve for design Y is generally of approximately optimalshape for alleviating threshold voltage roll-off in the immediatevicinity of the desired short-channel L_(DRC) operating value of 0.18μm. However, the V_(T)(L_(DR)) curve for design Y′ overshoots slightlyin the vicinity of the L_(DRC) value of 0.18 μm. This suggests that anupward doping distortion occurs in the vicinity of the 0.18-μm L_(DRC)value for design Y′. In general, nitriding the gate dielectric layerappears beneficial for achieving a V_(T)(L_(DR)) curve of largelyoptimal shape for alleviating short-channel threshold voltage roll-off,at least for n-channel IGFETs.

A split, referred to as process design Y″, of the second wafer lot wasemployed to investigate the effect of modifying the source/drain-haloanneal to include a furnace anneal for extended time at temperatureconsiderably below typical RTA temperature. As mentioned above, all ofthe annealing operations in the first lot were solely RTAs. Design Y″ ofthe second lot was processed at the conditions of design Y of the firstlot except that a furnace anneal was performed at 900° C. for 15 minafter the n-type main and lower-portion source/drain implants but beforethe p-type main and lower-portion source/drain implants.

FIG. 22 illustrates V_(T)(L_(DR)) variations for n-channel IGFETsmanufactured according to process designs Y and Y″. As FIG. 22indicates, inclusion of the extended-time furnace anneal in design Y″caused much of the undesirable short-channel threshold voltage roll-offto be brought back. Comparison of the V_(T)(L_(DR)) curves in FIG. 22suggests that more vertical dopant movement occurred with theextended-time furnace anneal of design Y″ than with thesource/drain-halo RTA of design Y, thereby causing the verticalchannel-center dopant profile in design Y″ to be relatively flat. Inturn, this then caused average net channel dopant concentration N_(C) inthe n-channel IGFETs of design Y″ to be reduced to a level at whichthreshold voltage roll-off occurs greatly in the immediate vicinity ofthe short-channel L_(DRC) value of 0.18 μm. It appears that actualchannel length L in design Y″ was shortened due to lateral movement ofthe n-type source/drain extension dopant during the extended-timefurnace anneal.

Additionally, the long-channel portions of the V_(T)(L_(DR)) curves forprocess designs Y and Y″ are very close to each other. Inclusion of theextended-time furnace anneal in design Y″ thus had little effect onthreshold voltage V_(T) in the long-channel regime.

The effect of varying the energy of ion implanting the p-type APT dopantwas examined with the third lot of wafers. In particular, the p-typeboron APT dopant for n-channel IGFETs was implanted at energies of 50,55, and 60 KeV in three splits of the third lot. The depth of themaximum value of the concentration of the APT dopant at the lowest APTimplantation energy, and thus at all three of the APT implantationenergies, occurred below the channel surface depletion region for eachof the n-channel IGFETs. The dosage of the boron APT dopant was 1×10¹³atoms/cm³ in all of the splits utilized to examine the effect of the APTenergy.

FIG. 23 depicts V_(T)(L_(DR)) variations for the splits at the APTimplantation energies of 50, 55, and 60 KeV. As the three V_(T)(L_(DR))curves in FIG. 23 indicate, threshold V_(T) at any given value of drawnchannel length I_(DR) decreases with increasing APT energy. Moreparticularly and unlike the threshold voltage control illustrated inFIG. 19a and achieved by changing the APT or/and halo dosages, the V_(T)control obtained by changing the APT implantation energy maintained thebasic shape of the V_(T)(L_(DR)) curve. Approximately the same amount ofthreshold roll-off relief is achieved at all three of the APT energies.

Also, the V_(T) control for n-channel IGFETs was nearly linear for theAPT implantation energy range from 50 KeV to 60 KeV. Threshold voltageV_(T) decreased approximately 8 mV for each KeV of increase in the APTenergy for n-channel IGFETs. It is expected that threshold voltage V_(T)of p-channel IGFETs manufactured according to the invention can likewisebe controlled in a nearly linear manner as a function of theimplantation energy of the n-type APT dopant.

The following phenomena appeared to be involved in controlling thresholdvoltage as a function of the energy of the APT implant. The p-type APTimplant for an n-channel IGFET is typically performed with boron, asmall atom, at a sufficiently high energy that the APT implantintroduces relatively few point defects into the IGFET's channel zone.Hence, the diffusion constant of the ion-implanted p-type halo dopantremains approximately constant, for a given halo dosage, as determinedby the amount and spatial distribution of the point defects created bythe halo implant at the given halo dosage. Changing the energy of theAPT implant does not significantly affect the distribution of the halodopant. For a given halo dosage, the halo dopant thus serves largely asa background parameter independent of the change in energy of the APTimplant.

Partially repeating what was stated above, threshold voltage V_(T) of asurface-channel enhancement-mode IGFET depends on the average net dopantconcentration in the channel surface depletion region through which thechannel extends during IGFET operation. Because the maximumconcentration of the APT dopant occurs below the channel surfacedepletion region in each of the present IGFETs, only part of the head ofthe APT implant is present in the channel surface depletion region.

The foregoing relationship between the channel surface depletion regionand the distribution of the implanted APT dopant can be seen in FIGS.24a and 24 b, particularly FIG. 24a, which present computer simulationsof vertical dopant profiles for long n-channel IGFETs of the third lotat p-type APT implantation energies of 50 and 60 KeV. FIG. 24a is takenalong the channel center where the APT dopant is present but(substantially) none of the p-type halo dopant is present. FIG. 24b istaken along a location near a source/drain zone where both the APT andhalo dopants are present.

Referring to FIG. 24a, the dopant variation along each of the twoleft-hand humps in FIG. 24a largely represents the dopant profile of theAPT implant in that portion of the channel zone of the associated IGFETwhose APT implant is performed at the indicated value of 50 or 60 KeV.As these portions of the dopant profile in FIG. 24a indicate, only partof the head of the APT implant is present in the channel surfacedepletion regions.

FIG. 24a also indicates that the head of the APT implant drops downwardas the energy of the APT implant increases from 50 KeV to 60 KeV.Consequently, the average net dopant concentration in the channelsurface depletion region decreases as the APT energy increases.Increasing the APT implantation energy thus causes threshold voltageV_(T) to decrease, typically in an approximately linear manner, for longn-channel IGFETs, and vice versa. Largely the same applies to the V_(T)magnitude of long p-channel IGFETs.

The same phenomena occurs in short-channel IGFETs except that the halodopant is present at the channel centers. Because the distribution ofthe halo dopant in the channel zone of a short-channel IGFET does notchange significantly due to a change in the implantation energy of theAPT dopant, increasing the energy of the APT implant causes themagnitude of threshold voltage V_(T) to similarly decrease, typically inan approximately linear manner, for a short-channel IGFET, and viceversa. Importantly, controlling threshold voltage V_(T) by adjusting theenergy of the APT implant is substantially independent of channel lengthL.

The ability to use the APT implantation energy for controlling thresholdvoltage V_(T) substantially independent of channel length L is highlyadvantageous. This capability enables threshold voltage V_(T) to beshifted upward or downward by approximately the same amount over theentire range of channel length L in an IC manufactured by a fabricationprocess which is fixed except for the energies of the APT implants.

Like-polarity IGFETs in different parts of an integrated circuit canalso be provided with different threshold voltages by utilizingdifferent values of the APT implantation energy for the differentlike-polarity IGFETs. For example, in an integrated circuit having amemory portion and a logic portion, the IGFETs in the memory portion canbe furnished with threshold voltages of high magnitude to achieve lowstandby power by performing memory APT implants at low energies into thememory portion while the logic portion is appropriately shielded from,or otherwise prevented from receiving, the memory APT implants. Thereverse of this procedure can be employed to furnish IGFETs in theologicportion with threshold voltages of low magnitude so as to achieve highdrive current and thus fast switching.

The effect of varying tilt angle α of the halo implants for bothn-channel and p-channel IGFETs was examined with the third lot ofwafers. FIG. 25 depicts V_(T)(L_(DR)) variations for n-channel IGFETs inthree splits at values of 20°, 30°, and 40° for tilt angle α. The p-typehalo dopant was boron in all three splits. The azimuthal angles for then-type halo implant were, as specified above, 33°, 123°, 213°, and 303°.

As indicated in FIG. 25, good short-channel threshold voltage roll-offcompensation occurred at the 40° tilt angle for n-channel IGFETs.Threshold voltage roll-off compensation disappeared when tilt angle αwas dropped to 30° or lower for n-channel IGFETs. The net result is thatlargely optimum threshold voltage roll-off compensation for n-channelIGFETs occurs when angle α is approximately 35°-40° at the specificp-type halo dosage used in the third wafer lot.

FIG. 26 depicts the V_(T)(L_(DR)) variations for p-channel IGFETs inthree splits at values of 20°, 30°, and 45° for tilt angle α. The n-typehalo dopant was arsenic for all three splits. As indicated in FIG. 26,short-channel threshold voltage roll-off compensation for p-channelIGFETs occurred with significant overshoot at the 45° tilt angle anddisappeared as tilt angle α dropped to 30° or lower. As with then-channel IGFETS, FIG. 26 suggests that largely optimum thresholdvoltage roll-off compensation for p-channel IGFETs occurs when angle αis approximately 35°-40° at the specific n-type halo dosage employed inthe third wafer lot.

The 5° width in the optimum tilt angle range for both n-channel andp-channel IGFETs provides sufficient room to allow for manufacturingtolerances and to avoid specific tilt angle values at which undesiredimplantation channeling occurs. While the value of tilt angle ax in therange of 35°-40° is optimum for the exemplary implementation of thepresent manufacturing used in the experimental program, another range oftilt angle ax, including a range width of 5° or more, may be optimum foranother implementation of the present manufacturing process.

The effect of utilizing high-energy/high-dosage wells was examined withthe fourth lot of wafers. FIGS. 27a and 27 b present computersimulations of vertical dopant profiles for two long n-channel IGFETS,one having a “normal” p-type well and the other having ahigh-energy/high-dosage p-type well referred to here simply as a “high”p-type well. FIG. 27a is taken along the channel center. FIG. 27b istaken along a location near a source/drain zone.

The normal p-type well is the p-type well employed and/or simulated inthe first, second, and third wafer lots. The maximum dopantconcentration of the high p-type well is greater than, and occurs deeperbelow the upper semiconductor surface, than the maximum concentration ofthe normal p-type well. The characteristics of a high well for ann-channel IGFET were simulated/chosen so as to have substantially thesame dopant profile along the upper semiconductor surface as thatpresent in the associated n-channel IGFET having a normal well. Use ofsuch a high well is important from the point of view of improving theability of an IC to avoid undesired latch-up.

FIG. 28 illustrates V_(T)(L_(DR)) variations for n-channel IGFETs in twosplits provided respectively with the normal and high p-type wells.There is a small, approximately 0.02-V, difference between theV_(T)(L_(DR)) curves for the splits with the normal and high p-typewells. This difference occurred even though the characteristics of thehigh well were chosen in accordance with the computer simulation to havesubstantially the same dopant profile along the upper semiconductorsurface as that achieved with the normal well. The V_(T) difference isattributed to insufficient simulation accuracy and can be compensatedfor by making a small increase, approximately 2.5 KeV, in the energy ofthe p-type APT implant for the split with the high p-type wells.

The effect of thinning the gate dielectric layer was investigated withthe fourth wafer lot for n-channel and p-channel IGFETs. One split wasprocessed to create gate dielectric layers of both n-channel andp-channel IGFETs at the nominal target thickness of 4 nm utilized in thefirst, second, and third lots. Another split was processed to provideboth n-channel and p-channel IGFETs with thinner gate dielectric layersat a reduced target thickness of 3.5 nm. The gate dielectric layers atboth target thicknesses were nitrided in the manner described above.

FIG. 29 depicts V_(T)(L_(DR)) variations for n-channel and p-channelIGFETs in the two splits where the target values of gate dielectricthickness t_(GD) respectively were 4 and 3.5 nm. As FIG. 29 indicates,thinning the gate dielectric layer did not have any significant effecton short-channel threshold voltage roll-off compensation. The IGFETs ofreduced gate dielectric thickness achieved largely the same amounts ofthreshold voltage roll-off relief as the IGFETs of the nominal gatedielectric thickness.

Threshold voltage V_(T) of the n-channel IGFETs at the reduced gatedielectric thickness was a small, approximately constant, amount lowerthan threshold voltage V_(T) of the n-channel IGFETs at the nominal gatedielectric thickness across the entire channel-length range examined inthe experiment. Threshold voltage V_(T) of the p-channel IGFETs atreduced dielectric thickness was similarly lower than threshold voltageV_(T) of the p-channel IGFETs at the nominal gate dielectric thicknessacross the entire channel-length range examined in the experiment. Thedifference in threshold voltage V_(T) was significantly greater for thep-channel IGFETs than for the n-channel IGFETS. This showed thatthinning the gate dielectric layer has a greater impact on p-channelIGFETs than on n-channel IGFETs.

In addition, the V_(T) difference between the p-channel IGFETs ofdifferent gate dielectric thickness increased slightly with increasingchannel length. Both this variation in the V_(T) difference and thegreater V_(T) difference for p-channel IGFETs than for n-channel IGFETsappear to be caused by boron in the p-type polysilicon gate electrodesof the p-channel IGFETs penetrating through the gate dielectric layersand into the underlying n-type channel zones. Boron penetration into thechannel zones of the p-channel IGFETs partially compensates the n-typedoping in the channel zones. As described above, nitrogen can beincorporated into the gate dielectric layers of p-channel IGFETs toreduce boron penetration through their gate dielectric layers.

FIG. 30 illustrates how extrapolated leakage current density I_(DSSX)varied with drive current density I_(DSAT) for the preceding n-channelIGFETs at the gate dielectric thicknesses t_(GD) of 4 and 3.5 nm. As inFIG. 19b, each point in FIG. 30 represents a different IGFET. Thefollowing leakage-drive characteristics at the desired short-channelL_(DR) operating value of 0.18 μm were extrapolated from FIG. 30 at thestandard value of 1 nA/nm for actual drive current density I_(DSS0):

I _(DSAT)=675 μA/μm at t _(GD)=3.5 nm

I _(DSAT)=625 μA/μm at t _(GD)=4 nm

The I_(DSAT) value for n-channel IGFETs at both gate dielectricthicknesses is greater than the I_(DSAT) value of 600 μA/μm achievedabove in process design Z of the first wafer lot.

The effect of reducing the time of the RTA in the source/drain-haloanneal was examined with the fourth lot for n-channel IGFETs. One splitwas processed with the nominal RTA time of 15 sec. used in the first,second, and third lots. Another split was processed with a reduced RTAtime of 12 sec.

FIG. 31a depicts V_(T)(L_(DR)) variations for n-channel IGFETs in thetwo splits where the RTA times were 15 and 12 sec. FIG. 31b illustratesthe corresponding I_(DSSX)(I_(DSAT)) graph. No significant difference isobserved between the n-channel IGFETs processed at the two different RTAtimes. This indicates that a substantial process margin is present inthe source/drain-halo RTA time for n-channel IGFETS. Reducing the RTAtime in the source/drain-halo anneal is important because undesirableboron penetration through the gate dielectric layers of p-channel IGFETsdecreases as the RTA time is reduced.

N-channel and p-channel IGFETs were provided with graded-junctioncharacteristics in an additional lot of wafers during the experimentalprogram. As with each of the first four lots, the wafers in theadditional lot were processed the same and thus largely simultaneouslyexcept for variations applied to the splits of the additional lot.Likewise, each split of the additional lot consisted of two or morewafers.

Subject to the variations applied to the different splits, the wafers inthe additional lot were processed as described above in connection withFIG. 16. Except for a baseline split, each source/drain zone of theIGFETs in the wafers of the additional lot was provided with a lowerportion underlying, and more lightly doped, than the very heavily dopedmain portion of that source/drain zone so as to create a gradedbody-source/drain pn junction. Boron, either in elemental form or in theform of boron difluoride, was the p-type dopant for all p-type ionimplantations in the additional lot.

P-channel IGFETs having graded junctions were compared to p-channelIGFETs having non-graded junctions in four splits, including theabove-mentioned baseline split, of the additional lot. The main portionsof the p-type source/drain zones of the p-channel IGFETs in these foursplits were formed by ion implanting boron in the form of borondifluoride at various values of ion implantation energy E_(BF2). For thebaseline split and one of the other three splits, implantation energyE_(BF2) was 25 KeV. E_(BF2) values of 20 and 15 KeV were respectivelyused for the remaining two splits.

The baseline split was not provided with graded-junctioncharacteristics. Accordingly, the source/drain zones of the p-channelIGFETs in the baseline split lacked lower portions more lightly dopedthan the main source/drain portions. The more lightly doped portions ofthe p-type source/drain zones of the graded-junction p-channel IGFETs inthe other three splits were formed by ion implanting boron in the formof elemental boron.

FIG. 32 illustrates how areal junction capacitance C_(J) varied as afunction of boron difluoride implantation energy E_(BF2) for p-channelIGFETs in the preceding four splits, including the baseline split. Arealcapacitance C_(J) is the capacitance per unit area along a pn junctionbetween a source/drain zone and the adjoining body material. As FIG. 32indicates, areal junction capacitance C_(J) was considerably higher forthe baseline split having the non-graded junctions than for the otherthree splits having graded junctions. This confirms that the use ofgraded junctions in p-channel IGFETs fabricated according to theinvention reduces the junction capacitance.

FIG. 32 also shows that areal junction capacitance C_(J) for thep-channel IGFETs in the three splits having graded junctions decreasedwith decreasing boron difluoride implantation energy E_(BF2). Thisarises because reducing implantation energy E_(BF2) decreases the range(average depth) of the ions of the boron difluoride implanted to formthe main portions of the p-type source/drain zones in thegraded-junction p-channel IGFETs. Since the more lightly doped lowerportions of the p-type source/drain zones define the body-source/drainpn junctions except along the source/drain extensions, decreasing therange of the boron difluoride ions implanted to form the mainsource/drain portions causes the average steepness of the gradedjunctions, and thus the junction capacitance, to decrease. Inasmuch asreducing the junction capacitance generally causes the switching speedto increase, FIG. 32 indicates that implantation energy E_(BF2) shouldbe made as small as reasonably feasible without causing fabricationcontrol problems.

The bottom thickness t_(SP) of the gate sidewalls spacers (70, 72, 90,92, 150, 152, 170, and 172) for the n-channel and p-channel IGFETs inthe preceding four splits of the additional wafer lot was approximately100 nm. The effect of reducing sidewall spacer thickness t_(SP) wasexamined by processing two splits of the additional lot to have a t_(SP)value of approximately 70 nm. The IGFETs in the two reduced-t_(SP)splits had graded-junction characteristics. Boron difluorideimplantation energy E_(EP2) was 25 KeV in one of the reduced-t_(SP)splits and 15 KeV in the other split.

FIG. 33 presents V_(T)(L_(DR)) variations for (a) graded-junction IGFETsin the earlier-mentioned split having the E_(BF2) value of 25 KeV andthe nominal t_(SP) value of 100 nm, (b) graded-junction p-channel IGFETsin the reduced-t_(SP) split having the E_(BF2) value of 25 KeV and thereduced t_(SP) value of 70 nm, and (c) graded-junction p-channel IGFETsin the reduced-t_(SP) split having the E_(BF2) value of 15 KeV and thereduced-t_(SP) value of 70 nm. These three V_(T)(L_(DR)) variations arerespectively labeled as curves A, B, and C in FIG. 33. FIG. 33 alsopresents the V_(T)(L_(DR)) variation, curve D, for graded-junctionn-channel IGFETs in the split having the nominal t_(SP) value of 100 nmalong with the V_(T)(L_(DR)) variation, curve E, for graded-junctionn-channel IGFETs in the two reduced-t_(SP) splits having the reducedt_(SP) value of 70 nm.

Referring first to curves A and B for which boron difluorideimplantation energy E_(BF2) is the same, 25 KeV, FIG. 33 shows thatreducing gate sidewall spacer thickness t_(SP) from 100 non to 70 nm inthe graded-junction p-channel IGFETs caused threshold voltage roll-offat short-channel length to increase. However, as indicated by curves Band C for which gate sidewall spacer thickness t_(SP) is the same, 70nm, much of the undesirable increase in threshold voltage roll-off waseliminated in graded-junction p-channel IGFETs by reducing implantationenergy E_(BF2) from 25 KeV to 15 KeV. In addition to again showing thebenefits of reducing implantation energy E_(BF2), this shows that thereis a significant process window with respect to parameters t_(SP) andE_(BF2) for alleviating threshold voltage roll-off in graded-junctionp-channel IGFETs.

Curves D and E for n-channel IGFETs at the respective nominal andreduced t_(SP) values are virtually identical. As shown by comparingcurve D or E for graded-junction n-channel IGFETs to curve A forgraded-junction p-channel IGFETs, the graded-junction n-channel IGFETsat either t_(SP) value had roughly the same amount of threshold voltageroll-off alleviation as the graded-junction p-channel IGFETs at thenominal t_(SP) value of 100 nm and the E_(BF2) value of 25 KeV. Hence,choosing appropriate parametric values to alleviate threshold voltageroll-off is quite important in designing the p-channel portions of thepresent fabrication process.

Shadowing Effects on Halo Implantation

The effect of shadowing during the halo doping operations needs to beconsidered when, as is preferably the situation, the halo dopings areperformed by angled ion implantation. Shadowing during angled haloimplantation can arise from either or both of two types of features thatprotrude significantly above the upper semiconductor surface: (a) gateelectrode structures formed with gate electrodes, or precursors to gateelectrodes, and any material situated on the top and side surfaces ofthe gate electrodes or their precursors and (b) masks, typicallyconsisting of photoresist, utilized to prevent halo dopant from beingimplanted into certain parts of the semiconductor material. If shadowingis not properly taken into account during angled halo implantation, halopockets may only be partially formed or, in extreme cases, may not beformed at all.

FIG. 34 presents a simplified model of how shadowing by a gate electrodestructure of one IGFET can affect angled implantation of a halo pocketof another IGFET referred to here as the halo-receiving IGFET. The IGFETwhose gate electrode structure poses shadowing concern is referred tohere as the halo-interfering IGFET because its gate electrode structureinvariably blocks some of the ions of the halo dopant from entering thesemiconductor material of the halo-receiving IGFET regardless of whetherthe blockage is great enough to damage the function of the halo pocketof the halo-receiving IGFET. The components, including the gateelectrode structures, of both the halo-receiving IGFET and thehalo-interfering IGFET are typically exposed through a common opening ina halo photoresist mask (not shown) situated over the semiconductorstructure under fabrication.

Both of the IGFETs under fabrication in the model of FIG. 34 arenormally of the same polarity type. In FIG. 34, the two IGFETs areillustrated as long n-channel devices. However, one or both of theIGFETs can be of the short n-channel type. Both of the IGFETs can alsobe p-channel IGFETs variously of the long-channel or short-channel type.Under certain circumstances, the halo-interfering IGFET can becomplementary to the halo-receiving IGFET.

The two exemplary n-channel IGFETs in the model of FIG. 34 are shown ashaving a common precursor source/drain extension 60EP/62EP.Alternatively, part of field-insulation region 44 can laterally separatesource/drain extension 60EP or 62EP of the halo-receiving IGFET fromsource/drain extension 60EP/62EP of the halo-interfering IGFET. Asimilar type of source/drain extension sharing or separation can arisewhen both IGFETs are p-channel devices. In the case where the two IGFETsare complementary to each other, field insulation 44 normally laterallyseparates the intended locations for the source/drain zones of the twoIGFETs.

The n-type source/drain extension implantation can, as indicated above,be done after the p-type halo implantation. If so, item 60EP/62EP inFIG. 34 represents the intended location for common source/drainextension 60EP/62EP.

Regardless of whether the halo-receiving and halo-interfering IGFETs arelong-channel or short-channel devices and regardless of whether they areof the same or opposite polarity type, each IGFET has a gate electrodestructure consisting at least of the IGFET's gate electrode, orprecursor to the gate electrode. When any material, such as electricalinsulation, is situated on the top and/or side surfaces of the gateelectrode or the precursor to the gate electrode during the haloimplantation, that material forms part of the gate electrode structure.Each of the n-channel IGFETs being fabricated in the model of FIG. 34has a gate electrode structure 68P/208 consisting of precursor gateelectrode 68P and seal-oxide layer 208. The gate electrode structuresare laterally separated by distance d_(G) in the direction along thechannel length of the halo-receiving IGFET.

In the model of FIG. 34, ions of the p-type halo dopant species arebeing implanted at tilt angle α to form a common precursor halo pocketportion 100P/102P for the two IGFETS. With the ions impinging from upperleft to lower right as shown in FIG. 34, the halo-receiving IGFET is theIGFET being formed at the right-hand side of the figure. When the ionsimpinge from upper right to lower left, the roles of the two IGFETs arereversed. The halo-interfering IGFET is then the IGFET being formed atthe left-hand side of FIG. 34. The model of FIG. 34 is simplified inthat the ions are assumed to impinge on the semiconductor body atopposite azimuthal angles of 0° and 180° relative to a reference lineextending in the longitudinal direction of the two IGFETS.

FIG. 34 illustrates what happens during the angled halo implantation asa result of reducing separation distance d_(G), i.e., bringing gateelectrode structures 68P/208 of the two IGFETs closer together, inlaying out the two IGFETS. Due to the angled implantation, shadowing bygate electrode structure 68P/208 of the halo-interfering IGFET, whetherit is being formed to the left as depicted in the part of the p-typehalo implantation presented in FIG. 34 or to the right as occurs in theother (unshown) part of the p-type halo implantation, progressivelyinhibits ions of the p-type halo dopant species from entering thesemiconductor structure as distance d_(G) is reduced. When distanced_(G) is made sufficiently small, shadowing by gate electrode structures68P/208 of the two IGFETs causes halo pocket 100P/102P to split intohalo pockets 102P and loop. Pocket 102P is a halo pocket for theleft-hand IGFET, while pocket 100P is a halo pocket for the right-handIGFET. As distance d_(G) is made even smaller, the lengths, in thelongitudinal direction, of pockets 100P and 102P are progressivelyreduced.

If the two IGFETs are laid out so that distance d_(G) between the gateelectrode structures drops down to or below a minimum value d_(Gmin),the halo implant is rendered inefficient or, in the worst case, useless.When distance d_(G) equals d_(Gmin), shadowing by gate electrodestructure 68P/208 of the halo-interfering IGFET to the left in the partof the halo implantation presented in FIG. 34 blocks the angled entry ofions into the semiconductor body to such a degree that halo pocket 100Pof the right-hand halo-receiving IGFET underlies its gate electrodestructure 68P/208 but does not extend significantly laterally across(the location for) source/drain extension 60EP/62EP. Halo pocket 100Pbecomes smaller and eventually disappears as distance d_(G) is reducedbelow d_(Gmin). Subject to the halo-interfering/halo-receiving rolesbeing reversed for the two IGFETs, the same occurs to halo pocket 102Pof the left-hand IGFET.

While ions of the n-type halo dopant species are assumed to impinge onthe semiconductor body at the opposite azimuthal angles of 0° and 180°relative to a reference line extending in the IGFETs' longitudinaldirection during the two respective parts of the n-type haloimplantation in the simplified model of FIG. 34, the ions typicallyimpinge on the semiconductor body at average azimuthal angles thatrespectively differ significantly from 0° and 180°. Let β generallyrepresent the average azimuthal angle, as measured from the indicatedlongitudinal reference line in a plane extending generally parallel tothe upper semiconductor surface, at which the ions impinge on thesemiconductor body during each part of the n-type halo implantation.General azimuthal angle β then has two values, one for each part of then-type halo implantation. Assume that the two β values differ by 180° asis typically the case.

Upon taking azimuthal angle β into account for the 180°azimuthal-difference assumption given in the previous paragraph, minimumvalue d_(Gmin) is given approximately by the following relationship:

d _(Gmin)=(h _(G) +t _(GD) +R _(Halo))tan α·|cos β|  (18)

where h_(G) is the height of the gate electrode structure (68P/208Phere) of the halo-interfering IGFET above the gate dielectric layer (66here), t_(GD) is the thickness of the gate dielectric layer (66 here) ofthe halo-receiving IGFET, and R_(Halo) is the vertical range (averagedepth) of the ions of the halo dopant (p-type halo dopant here) into thesemiconductor body. For Eq. 18, it is immaterial whether dielectricmaterial (210 here) is, or is not, present along the upper semiconductorsurface portion where the halo dopant enters the semiconductor body.Also, it is immaterial which of the two β values is used in Eq. 18 since|cos(180°+β)| equals |cos β|.

The factor (t_(GD)+R_(Halo))tan α·|cos β| in Eq. 18 accounts for thelateral thickness thio of halo pocket loop directly after the haloimplantation at a depth below the upper semiconductor surface equal tovertical range R_(Halo) of the halo dopant. Lateral thickness t_(Halo)here is the distance by which halo pocket 100P extends from precursorsource/drain extension 60EP/62EP laterally below right-hand gateelectrode structure 68P/208 at a depth equal to R_(Halo). Atoms of thehalo dopant are present across all of lateral halo thickness t_(Halo).If factor (t_(GD)+R_(Halo)) tan α·|cos β| were not present in Eq. 18,atoms of the halo dopant might be present to some degree below gateelectrode structure 68P/208 of the right-hand IGFET in FIG. 34 but wouldnormally not be present at the desired concentration across all ofthickness t_(Halo). The same applies to halo pocket 102P of theleft-hand IGFET.

In cases where part of the p-type halo implantation is performed at twoor more azimuthal angles which differ from 0° by less than 90°, thevalues of angles α and β for insertion into Eq. 18 to determine minimumvalue d_(Gmin) are those which yield the highest value for the factortan α·|cos β|. If tilt angle α is the same for the two or more azimuthalangles but they differ from 0° by different amounts, value d_(Gmin) isdetermined by the azimuthal angle that differs the least from 0°, i.e.,the smallest azimuthal angle. Similarly, when part of the p-type haloimplantation is conducted at two or more azimuthal angles which differfrom 180° by less than 90°, the values of angles α and β for insertioninto Eq. 18 are those for which factor tan α·|cos β| yields the lowestvalue. If the two or more azimuthal angles differ from 180° by differentamounts while tilt angle α is constant, value d_(Gmin) is determined bythe azimuthal angle that differs the least from 180°.

As indicated above, the halo implant will not perform the desiredfunction if distance d_(G) between the gate electrode structures is lessthan d_(Gmin). In other words, separation distance do and minimum valued_(Gmin) should satisfy the relationship:

d _(G) ≧d _(Gmin)  (19)

where value d_(Gmin) is given by Eq. 18. Relationship 19 is employed tosatisfy layout design rules for the minimum distance between gateelectrodes of adjoining IGFETs in the direction of the length of eachIGFET. Relationship 19 applies to the situation in which the adjoiningIGFETs are of the same polarity type and have their component materialexposed through a common opening in a photoresist mask. In view of whatis said above, relationship 19 can also apply to the situation in whichthe adjoining IGFETs are of opposite polarity type and in which the haloimplantation is performed selectively without the use of a photoresistmask or equivalent.

Satisfying relationship 19 applies regardless of whether the twoadjoining IGFETs are laid out to have a common source/drain zone or tohave adjoining source/drain zones laterally separated by part of fieldinsulation 44. However, relationship 19 is often inherently satisfiedwhen part of field insulation 44 laterally separates otherwise adjoiningsource/drain zones of the two adjacent IGFETs. Hence, specific attentionmust be paid to satisfying relationship 19 when the two adjacent IGFETsare laid out to have a common source/drain zone.

FIG. 34 illustrates the situation in which the gate electrode structuresof the two adjacent IGFETs extend generally parallel to each other. Thisis the most critical situation in which relationship 19 needs to besatisfied because, if separation distance d_(G) is less than d_(Gmin),ions of the halo dopant are normally prevented from passing into thesemiconductor material below the gate electrode structure of thehalo-receiving IGFET along more of the width of the gate electrodestructure of the halo-receiving IGFET than when the gate electrodestructure of the two IGFETs are at a non-zero angle to each other.Nonetheless, if distance d_(G) is less than d_(Gmin), at least partialblockage of halo ions into the semiconductor material below the gateelectrode structure of the halo-receiving IGFET can occur when the gateelectrode structures of the two. IGFETs are at the most severe angle,90°, to each other. Consequently, satisfying relationship 19 isimportant regardless of the angle between the gate electrode structures.

Turning to shadowing effects that can arise with implantation masks suchas those made of photoresist, FIG. 35 presents a simplified model of howshadowing by a photoresist mask can affect angled implantation of a halopocket of an IGFET (again) referred to here as the halo-receiving IGFET.FIG. 36 represents a layout view of the model of FIG. 35.

The halo-receiving IGFET in the model of FIGS. 35 and 36 is, forexample, illustrated as a long n-channel device (40) but can as well bea short n-channel device (42). In either case, the photoresist mask isphotoresist 220. Alternatively, the halo-receiving IGFET can be ap-channel device of the long-channel type (120) or the short-channeltype (122). In this case, the photoresist mask is photoresist 212. Thehalo-receiving IGFET is typically symmetrical but, as discussed below,can be asymmetrical.

In the model of FIGS. 35 and 36, ions of the p-type halo dopant arebeing implanted at tilt angle α through an opening 230 in photoresistmask 220 to form halo pocket portion 102P of the halo-receiving IGFET.Similar to the model of FIG. 34, the model of FIGS. 35 and 36 issimplified in that the ions of the halo dopant species are assumed toimpinge on photoresist mask 220 and the underlying semiconductor body atopposite azimuthal angles of 0° and 180° relative to a reference lineextending in the longitudinal direction of the halo-receiving IGFET.Item d_(M) in FIGS. 35 and 36 is the distance extending from the side ofgate electrode structure 68P/208 overlying the intended location forhalo pocket 102P in the longitudinal direction across the directlyadjoining source/drain location to the nearest edge of photoresist mask220. Although FIG. 35 illustrates precursor drain/extension 62EP asbeing present during the p-type halo implantation, extension 62EP can becreated after performing the p-type halo implantation. In that case,item 62P in FIG. 35 represents the intended location of extension 62EP.

FIG. 35 illustrates what happens during angled halo implantation as aresult of reducing separation distance d_(M), i.e., bringing photoresistmask 220 closer to gate electrode structure 68P/208 of thehalo-receiving IGFET, during the fabrication of photoresist 220. Due tothe angled implantation, shadowing by photoresist 220 starts tosignificantly inhibit ions of the p-type halo dopant from entering themonosilicon semiconductor body to form halo pocket 102P and, inparticular, to form the part of pocket 102P below gate electrodestructure 68P/208, when distance d_(M) is at or below a minimumtolerable value d_(Mmin). If photoresist 220 is configured so thatdistance d_(M) is less than minimum value d_(Mmin), the haloimplantation becomes inefficient or, in the worst case, useless.

More particularly, FIG. 35 illustrates halo pocket 102P as extendingfully across (the location for) source/drain extension 62EP. Dependingon the value of tilt angle α and the horizontal and vertical dimensionsof the features along the upper semiconductor surface, thisconfiguration of pocket 102P can arise solely from the illustrated ionimplantation from upper right to lower left or from both theimplantation from upper right to lower left and the associatedimplantation from upper left to lower right. In any event, shadowing byphotoresist mask 220 and/or gate electrode structure 68P/208 typicallycauses halo pocket 102P to split into a left-hand portion and aright-hand portion when distance d_(M) is made sufficiently small. Theright-hand portion is largely electrically inconsequential in the modelof FIG. 35 and is not mentioned further here. The left-hand portion is,for convenience, referred to here as halo pocket 102P.

Similar to what was said above about the model of FIG. 34, furtherreduction in distance d_(M) causes the length, in the longitudinaldirection, of halo pocket 102P to be progressively reduced. Whendistance d_(M) equals d_(Mmin), shadowing by photoresist mask 220 blocksthe angled entry of ions into the semiconductor body to such an extentthat halo pocket 102P underlies gate electrode structure 68P/208 butdoes not extend significantly laterally across (the location for)source/drain extension 62EP. Reducing distance d_(M) below d_(Mmin)causes halo pocket 102P to become smaller and eventually disappear.

Upon taking azimuthal angle P into account for the 180°azimuthal-difference assumption mentioned above in connection with thegate-shadowing model of FIG. 34, minimum value d_(Mmin) is givenapproximately by the following relationship:

d _(Mmin)=(h _(M) +t _(GD) +R _(Halo))tan α·|cos β|  (20)

where h_(M) is the height (thickness) of the photoresist mask (220 here)above the gate dielectric layer (66 here). As with Eq. 18, it isimmaterial which of the two 180° different β values is used in Eq. 20since |cos(180°+β)| equals |cos β|. Similar to what was said above inconnection with Eq. 18, the factor (t_(GD)+R_(Halo))tan α·|cos β| in Eq.20 accounts of the lateral thickness t_(Halo) of halo pocket 102P at adepth below the upper semiconductor surface equal to vertical halo rangeR_(Halo) so that atoms of the halo dopant are present across all ofthickness t_(Halo). The comments made above about what values of anglesα and β are to be utilized to determine minimum value d_(Gmin) from Eq.18 when part of the p-type halo implantation is performed at two or moreazimuthal angles which differ from 0° by less that 90° or when part ofthe p-type halo implantation is performed at two or more azimuthalangles which differ from 180° by less than 90° apply to determiningminimum value d_(Mmin) from Eq. 20.

Eq. 20 applies, of course, to forming any halo pocket described above.Separation distance d_(M) is then generally the distance from the gateelectrode structure of the halo-receiving IGFET across the location foreach source/drain zone of the halo-receiving IGFET to the photoresistmask used in the halo implantation.

As indicated above, the halo implantation will not yield the desiredbenefit if separation distance d_(M) is less than d_(Mmin).Alternatively stated, separation distance d_(M) and minimum valued_(Mmin) should satisfy the relationship:

d _(M) ≧d _(Mmin)  (21)

where value d_(Mmin) is given by Eq. 20. Satisfying relationship 21provides one constraint in locating the mask opening that exposes thelocation where the halo-receiving IGFET is being formed.

The component material of two or more like-polarity IGFETS, whosechannel lengths are situated essentially end-to-end (aside from lateralseparation between the channel lengths) so that the IGFETs will begenerally in a straight line, may be exposed through a common opening ina photoresist mask during halo implantation of the IGFETs. The IGFETsmay be laid out to variously have one or more common source/drain zonesor to variously have source/drain zones laterally separated by one ormore parts of field insulation 44. In such cases, relationship 20 isapplied to the component material of the IGFET at each end of the lineof IGFETs.

As described below in connection with FIGS. 41a, 41 b, 42 a, and 42 b,an asymmetrical long-channel IGFET configured in accordance with theinvention may have only a single halo pocket rather than two halopockets. The single halo pocket is formed by implanting ions of the halodopant in such a manner that the halo ions pass directly by only one ofthe two opposite transverse sides of the gate electrode structure of thelong-channel IGFET. A halo pocket for an asymmetrical short-channelIGFET configured in accordance with the invention is formed in the sameway. Although the halo pocket for the long-channel IGFET adjoins onlyone of its source/drain zones, the halo pocket for the short-channelIGFET normally adjoins both of its source/drain zones due to the reducedchannel length.

The asymmetrical long-channel and short-channel IGFETs can be fabricatedin the manner generally described in Wang et al, U.S. patent applicationSer. No. 09/540,734, cited above, by appropriately controlling thephotoresist mask through which the halo dopant is introduced into thesemiconductor body. For this purpose, relationship 21 is applied to onlyone of the transverse sides of the gate electrode structure of each suchasymmetrical IGFET. The complement of relationship 21, excluding thefactor (t_(GD)+R_(Halo))tan α−|cos β|, is applied to the othertransverse side of the IGFET's gate electrode structure so that haloions do not significantly pass by that transverse side of the gateelectrode structure and into the IGFET's channel zone subject toseparation distance d_(M) not being so small along that transverse sideof the gate electrode structure as to significantly impair the passageof ions of the n-type source/drain extension dopant directly by thattransverse side of the gate electrode structure and into the underlyingmonosilicon.

The model of FIGS. 35 and 36 also facilitates understanding whatprecautions need to be taken with a photoresist mask to avoid havingangled halo implantation detrimentally affect the manufacture of anotherIGFET adjacent to, and of opposite polarity type to, the halo-receivingIGFET. As depicted in FIGS. 35 and 36, this situation can arise when thehalo-receiving IGFET and the other IGFET, referred to here as thecomplementary IGFET, are arranged so that (aside from lateral separationbetween the channel lengths), their channel lengths are situatedessentially end-to-end. Part of field insulation 44 laterally separatessemiconductor material of the two IGFETs.

Inasmuch as the halo-receiving IGFET is an n-channel device in theexemplary model of FIGS. 35 and 36, the complementary IGFET is ap-channel IGFET and is illustrated as being of the long-channel type(120) but can be of the short-channel type (122). The polarities of theIGFETs can, of course, be reversed.

When the channel lengths of the halo-receiving and complementary IGFETsare arranged in the foregoing end-to-end manner and thus extend in thesame direction, separation distance d_(M) must be less than a maximumtolerable value d_(Mmax) to avoid having the halo ions enter the channelzone of the complementary IGFET and damage its performance. Inparticular, the absolute value of threshold voltage V_(T) of thecomplementary IGFET will drop below the expected value if distance d_(M)is greater than d_(Mmax).

Maximum value d_(Mmax) is given approximately as:

d _(Mmax)=2c+i+R _(Rxt) tan α·|cos β|  (22)

where c is the distance from the gate electrode structure of thehalo-receiving IGFET across the intended or actual location for one ofits source/drain zones to field insulation 44 in the direction along thechannel lengths, i is the width of the intervening part of fieldinsulation 44 in the direction along the channel lengths, and R_(Rxt) isthe vertical range (average depth) of the ions of the source/drainextension dopant for the complementary IGFET.

As mentioned above, the magnitude of threshold voltage V_(T) of thecomplementary IGFET will be undesirably low if separation distance d_(M)is greater than d_(Mmax). Hence, separation distance d_(M) and maximumvalue d_(Mmax) should satisfy the relationship:

d _(M) ≦d _(Mmax)  (23)

Combining relationships 21 and 23 yields:

d _(Mmin) ≦d _(M) ≦d _(Mmax)  (24)

where values d_(Mmin) and d_(Mmax) are respectively given by Eqs. 20 and22. Relationship 24 is utilized in laying out both the halo-receivingand complementary IGFETs and in determining the location of the maskopening so that the halo pocket for the halo-receiving IGFET is createdproperly without causing the magnitude of threshold voltage V_(T) of thecomplementary IGFET to drop below the expected value.

Dopant Diffusion Effects

The alleviation of short-channel threshold voltage roll-off in IGFETsconfigured according to the invention has been explained above in termsof locally increasing average net channel dopant concentration N_(C).While this explanation does conform to experimental observation,threshold voltage V_(T) of an IGFET such as one of the present IGFETsdepends specifically on the characteristics at, or very close to, theupper semiconductor surface. When semiconductor dopant of the sameconductivity type as the body material of the channel zone diffuses tothe upper channel surface to raise the net channel surface dopantconcentration, threshold voltage V_(T) normally generally increasesaccording to Eq. 1 above.

By furnishing one of the present IGFETs with one or more halo pockets,the average net channel dopant concentration is increased. Independentlyof this, an increased amount of dopant of the same conductivity type asthe IGFET's channel zone diffuses to the upper semiconductor surface.This diffusion, which causes the net channel surface dopantconcentration to increase, is a primary factor behind the reverseshort-channel effect reported for non-haloed IGFETs.

IGFETs fabricated in accordance with the invention display a reverseshort-channel effect. That is, the magnitude of threshold voltage V_(T)of like-polarity long-channel IGFETs manufactured according to the same,or largely the same, fabrication process of the invention decreases withincreasing channel length. This is illustrated by the roll-offcompensating curves in FIGS. 6 and 8 for n-channel IGFETs at channellength values of up to 1.4 μm at the low end of the long-channel regime.The reverse short-channel effect in the present IGFETs appears to arisefrom a combination of (a) upper-surface dopant pile-up that occursindependent of halo dopant and (b) additional upper-surface dopantpile-up due to halo dopant.

The reverse short-channel effect in the present IGFETs appears to gobeyond the limits of normal diffusion. With reference to FIGS. 18, 22,and 23, the reverse short-channel effect in IGFETs manufacturedaccording to the invention is experimentally observed at a channellength of 10-20 μm. On the other hand, the value of channel lengthneeded for halo pockets 100 and 102 in long-channel IGFET 40 to belaterally separated is, at most, 1 μm if only normal diffusion isinvolved in forming pockets 100 and 102. This indicates that dopantredistribution occurs over distances much greater than that indicated bynormal diffusion at the reduced thermal budget preferably employed infabricating the present IGFETs. In other words, the p-type halo dopantin pockets 100 and 102 appears to move closer to the channel center thanwhat would arise from normal diffusion.

FIGS. 37 and 38 present a model that facilitates understanding therelationship between the reverse short-channel effect in the presentIGFETs and dopant redistribution in their channel zones, assuming thatsuch dopant redistribution takes place over the longer distancesmentioned above. FIG. 37 illustrates a vertical cross section of half ofthe structure of each of three symmetrical n-channel IGFETs manufacturedaccording to the same, or largely the same, implementation of thepresent fabrication process so as to be of three different channellengths. FIG. 38 depicts the dopant profile along the uppersemiconductor surface in the direction along the channel length for eachof the IGFETs in FIG. 37.

The three IGFETs in FIG. 37 implement long n-channel device 40 and arerespectively labeled as n-channel IGFETs 40A, 40B, and 40C alongvertical planes extending through their channel centers. Channel lengthL decreases in going from IGFET 40A to IGFET 40C. The half structures ofIGFETs 40A, 40B, and 40C respectively have halo pockets 10A, 100B, and100C which implement halo pocket 100. Since IGFETs 40A-40C aremanufactured according to the same, or largely the same, processconditions, halo pockets 100A-100C each contain largely the same amountof p-type halo dopant per unit drain width.

FIG. 38 specifically shows how net dopant concentration N_(N) varieswith longitudinal distance into the channel zone along the uppersemiconductor surface for each of IGFETs 40A-40C. As indicated above,enhanced dopant diffusion is assumed to occur in each of IGFETs 40A-40C.Curve segments 100A*, 100B*, and 100C* respectively represent the netdopant concentrations of halo pockets 100A-100C along the uppersemiconductor surface. Curve segment 64*, which splits into curvesegments 100A*-100C*, represents the net dopant concentration of channelzone 64 along the upper semiconductor surface. Curve segment 64* has apedestal caused by imposition of the p-type APT dopant on the backgroundchannel doping level. Inasmuch as the background channel doping level isnormally much less than the surface doping level of the APT dopant, thepedestal in curve segment 64* constitutes the surface doping level ofthe APT dopant. Curve segments 60E* and 60M* in FIG. 38 respectivelyrepresent the net surface dopant concentrations of source/drainextension 60E and main source/drain portion 60M.

IGFET 40A represents a device of great channel length. In IGFET 40A, thep-type halo dopant initially located along each source/drain extension60E or 62E spreads freely towards the channel center. Curve segment100A*, representing the net surface dopant concentration of halo pocket100A, is shaped like an asymmetric bell that ends at the pedestal levelof the APT implant in moving towards the channel center. Thresholdvoltage V_(T) of IGFET 40A is determined from Eq. 1 subject to averagenet channel dopant concentration N_(C) in Eq. 1 being replaced with thepeak value of curve segment 100A*. The peak concentration value isemployed because it generally presents the higher barrier to currentflow and therefore controls threshold voltage V_(T).

As channel length L is reduced in going from IGFET 40A through IGFET 40Bto IGFET 40C while still being in the long-channel regime, the fixedamount of p-type halo dopant per unit drain width at each end of channelzone 64 is confined within a progressively smaller volume. Hence, thebell shape of the halo dopant that defines halo portion 100 compresseslaterally and reaches a higher peak as shown by curve segments 100B* and100C* in FIG. 38. Using Eq. 1 to determine long-channel thresholdvoltage V_(T0) with the peak values of curve segments 100B* and 100C*being substituted for average net channel dopant concentration N_(C) inFIG. 1, threshold voltage V_(T) increases in going from IGFET 40Athrough IGFET 40B to IGFET 40C. Alternatively stated, threshold voltageV_(T) decreases with increasing channel length, thereby evincing thereverse short-channel effect.

In going from shortest long-channel IGFET 40C to short-channel IGFET 42(not shown here), halo pocket 100C merges with its twin 102C (not shown)to form extended halo pocket 104 of IGFET 42 with halo dopant beingpresent at the channel center in IGFET 42, the net surface dopantconcentration along the channel center in IGFET 42 is greater than thepedestal level of the APT dopant. Because even less volume is availableto contain the fixed amount of p-type halo dopant per unit drain widthin IGFET 42, the peak of the net surface dopant concentration inextended halo pocket 104 is greater than the peak of curve segment 100C*for IGFET 40C. The overall value of average net channel dopantconcentration N_(C) thereby rises sufficiently to alleviateshort-channel threshold voltage roll-off.

The mechanism responsible for enhancing the lateral spreading of thehalo dopant appears to be pair-enhanced diffusion which takes placeduring the high-temperature transient of the RTA in thesource/drain-halo anneal. Consider the situation in which source/drainextensions 60E and 62E are formed with arsenic. Prior to implantingarsenic to form extensions 60E and 62E, the monosilicon crystal belowgate electrode 68P contains the following atomic species: (a) siliconatoms on crystal sites, (b) boron dopant atoms on substitutional sites,(c) silicon vacancies, and (d) silicon interstitials. Each of these fouratomic species is present at its own level of equilibrium concentration.

Arsenic is a relatively large atom compared to boron. During the arsenicimplantation to form source/drain extensions 60E and 62E, arsenic ionsknock silicon atoms off their crystal sites. Silicon interstitials arethereby created at a level above the initial silicon interstitial level.In general, crystal damage caused by ion implantation increases thecapability of diffusion to occur in the crystal because there are moresites for atoms to diffuse to.

During the initial part of the source/drain-halo RTA, siliconinterstitials pair up with boron atoms to form (siliconinterstitial)-(boron substitutional) pairs. The (siliconinterstitial)-(boron substitutional) pairs diffuse through the crystalgenerally towards the upper semiconductor surface where a large amountof imperfections can be tolerated. Eventually, the interstitial-boronpairs break apart. The boron atoms generally settle at substitutionalsites within the crystal while the silicon interstitials generallyconcentrate at the upper semiconductor surface.

A similar type of pair-enhanced diffusion accounts for additionallateral spreading in long p-channel IGFET 120 where the dopant atoms inthe crystal lattice are n-type atoms such as phosphorus. Ions of thep-type dopant, normally boron, implanted to form source/drain zones 140and 142 have sufficient energy to knock silicon atoms off their crystalsites and create silicon interstitials. During the source/drain-haloanneal, silicon interstitial-n-type dopant substitutional pairs areformed and diffuse through the crystal in the manner described above.

For pair-enhanced diffusion, the flux (current density) {right arrowover (J)}_((AI)) of the pairs AI of substitutional halo dopant atom Aand interstitial atom I is given approximately as follows:

{right arrow over (J)} _((AI)) =−D _((A0))∇(KC _((A)) ^(TOT)), K>1  (25)

where D_((A0)) is the equilibrium diffusion constant of halo dopantatoms A, K is a dimensionless spatially dependent parameter whichdescribes the degree of local diffusion enhancement, and C_((A)) ^(TOT)is the total single/paired concentration of halo dopant atoms A in thecrystal. As indicated above, spatially dependent parameter K is greaterthan 1. If parameter K equaled 1, Eq. 25 would describe normaldiffusion.

To simplify the analysis, assume that spatial parameter K isapproximately constant. Eq. 25 than becomes:

{right arrow over (J)} _((AI)) =−D _((A0)) K∇C _((A)) ^(TOT) , K≦1  (26)

Eqs. 25 and 26 both indicate that, for pair-enhanced diffusion to takeplace, there must be a strong gradient in the total dopant concentrationat the location where pair-enhanced diffusion is to occur. Also,pair-enhanced diffusion requires a source of silicon interstitials.

FIG. 39 presents a model of how pair-enhanced boron diffusion in longn-channel IGFET 40 accounts for greater lateral boron halo diffusion. Insource/drain extensions 60B and 62E which respectively adjoin halopockets 100 and 102 in IGFET 40, ion implantation of the n-typesource/drain extension dopant, typically arsenic, causes extensions 60Eand 62E to have a high concentration of silicon interstitials. Then-type source/drain extension dopant in extensions 60E and 62E has astrong vertical gradient towards the upper semiconductor surface. Thesilicon interstitials, either alone or paired with boron atoms, movetoward the upper semiconductor surface.

In the channel-zone material along the pn junction between upperbody-material portion 52 and each source/drain extension 60E or 62E, thesilicon interstitials move vertically to available upper semiconductorsurface sites, including those located laterally far into the intendedchannel-zone location and thus far from the junction betweenbody-material portion 52 and each extension 60E or 62E. This leads to anangled flux of silicon interstitials towards the channel center andupper surface of the intended channel zone as shown in FIG. 39. Due tothe p-type halo and APT implants, an angled flux of boron halo dopantoccurs in generally the same direction and towards the same location.Pair-enhanced diffusion of boron thereby takes place over abnormallylong distances towards the channel center and upper semiconductorsurface.

Near the channel center of IGFET 40, normal diffusion occurs with thehead of the p-type APT dopant. Since the APT dopant is implantedrelatively deep into p-type body material 50 and since the normal APTdopant atoms, boron, are relatively small, the APT implant produces arelatively small amount of silicon interstitials. The APT boron dopantnear the channel center undergoes relatively little pair-enhanceddiffusion. Referring to FIG. 38, the dopant profile along the uppersemiconductor surface near the channel center reaches the pedestal levelproduced by the APT dopant.

For comparison purposes, FIGS. 40a and 40 b present a computersimulation of the vertical dopant profiles for a short n-channel IGFETof process design Z in the first lot of experimental wafers. FIG. 40a istaken along the channel center. FIG. 40b is taken along a location neara source/drain zone. Since the IGFET in FIGS. 40a and 40 b is ashort-channel device corresponding to short n-channel IGFET 42, the haloand APT dopants are both present at the channel center and along thesource/drain zone location.

FIGS. 40a and 40 b illustrate how the p-type halo doping operation andthe source/drain-halo RTA affect the vertical dopant profile in theshort n-channel IGFET of design Z. The curves marked “Initial” representthe vertical dopant profiles directly after formation of seal-oxidelayer 208 and thus prior to the introduction of the p-type halo dopantand the n-type source/drain extension dopant into the semiconductorbody. The curves marked “Final” represent the vertical dopant profilesdirectly after the source/drain-halo RTA and thus after the p-type halodoping operation. As FIG. 40a indicates, the net dopant concentration inthe channel zone along the upper semiconductor surface is roughly 4×10¹⁷atoms/cm³ at the channel center and greater near each source/drain zone.This enables threshold voltage V_(T) of the simulated n-channel IGFET tobe approximately 0.4 V.

Silicon interstitials can be produced by mechanisms other thanintroducing semiconductor dopant into a semiconductor body. For example,atoms of the semiconductor material, silicon here, can be implanted intothe silicon semiconductor body to form silicon interstitials. Theadvantage of using silicon atoms, or other non-dopant atoms, to producesilicon interstitials is that the total content of the dopant in thecrystal is not changed. The energy and dosage of the silicon implantsare determined in accordance with the parameters of the APT implant.

Asymmetrical IGFETs

The fabrication process of FIG. 16 can be directly utilized, or readilymodified, to fabricate asymmetrical IGFETs according to the invention.Although the source/drain zones of a symmetrical IGFET can typicallyswitch efficiently between functioning as source and functioning asdrain, one of the source/drain zones of an asymmetrical IGFET normallyfunctions permanently as the source while the other source/drain zonenormally functions permanently as the drain. An asymmetrical IGFETtypically provides higher drive current at the same leakage current thana comparable symmetrical IGFET.

FIGS. 41a and 41 b respectively illustrate asymmetrical n-channel IGFETs240 and 242 configured according to the invention. IGFET 240 is along-channel device arranged the same as long-channel IGFET 40 exceptthat the p-type halo implant for IGFET 240 is performed in such a waythat IGFET 240 has only a single halo pocket portion 244 situated alongsource/drain zone 62 rather than symmetrical halo pocket portions 100and 102 situated respectively along source/drain zones 60 and 62.Similarly, IGFET 242 is a short-channel device arranged the same asshort-channel IGFET 42 except that the halo implant for IGFET 242 isperformed so that IGFET 242 has a single asymmetrical halo pocketportion 246 situated primarily along source-drain zone 82 rather thanextended halo pocket portion 104 situated symmetrically alongsource/drain zones 80 and 82. Halo pocket 246 extends fully acrosschannel zone 84 in short-channel IGFET 42 and adjoins source/drain zone80 along a smaller area than where halo pocket 246 adjoins source/drainzone 82.

Asymmetrical IGFETs 240 and 242 may be part of the same overallsemiconductor structure and thus are manufactured according to thesame-fabrication process. Alternatively, IGFETs 240 and 242 may be partof separately fabricated, and therefore different, semiconductorstructures. In the latter case, largely the same fabrication process isemployed to manufacture IGFETs 240 and 242. The semiconductor structureor structures containing IGFET 240 and/or IGFET 242 typically containsp-channel IGFETs, including one or more asymmetrical p-channel IGFETsconfigured in a similar, but complementary, manner to n-channel IGFETs240 and 242. Also, the semiconductor structure or structures containingIGFET 240 and/or IGFET 242 may include one or more of symmetricaln-channel IGFETs 40 and 42 and symmetrical p-channel IGFETs 120 and 122.

The net dopant concentration of channel zone 64 in long-channel IGFET240 reaches a local surface minimum along the upper semiconductorsurface in the longitudinal direction at a location between source/drainzones 60 and 62. In this regard, FIG. 42a presents an example of how netdopant concentration N_(N) in IGFET 240 varies along the uppersemiconductor surface in the longitudinal direction. Aside from thedifference caused by the presence of single halo pocket 244 rather thansymmetrical halo pockets 100 and 102, the surface dopant profile in FIG.42a for IGFET 240 is substantially the same as the surface dopantprofile in FIG. 10a for IGFET 40. Horizontal distance x in FIG. 42a ismeasured the same as in FIG. 10a. Curve segment 244* in FIG. 42arepresents the net dopant concentration in halo pocket 244 along theupper semiconductor surface.

The solid-line portion of curve segment 64* in FIG. 42a represents thenet surface dopant concentration in the right half of channel zone 64from the channel center to source/drain zone 62. Part of the solid-lineportion of curve segment 64* is also marked 244* since halo pocket 244lies in the right half of channel zone 64. The dashed-line portion ofcurve segment 64* in FIG. 42a represents the net surface dopantconcentration in the left half of channel zone 64 from the channelcenter to source/drain zone 60.

The longitudinal local surface minimum in the net dopant concentrationof channel zone 64 for IGFET 240 occurs in an upper semiconductorsurface portion situated between halo pocket 244 and source/drain zone60. As indicated by the flat portion of the solid-line and dashed-lineportions of curve segment 64* in FIG. 42a, the net dopant concentrationof channel zone 64 for IGFET 240 is approximately constant at thelongitudinal local surface minimum along a non-zero portion of animaginary line (not shown) extending longitudinally from the channelcenter along the upper semiconductor surface toward each source/drainzone 60 or 62. Accordingly, the net dopant concentration of channel zone64 in IGFET 240 is approximately constant at the longitudinal localsurface minimum along a non-zero portion of a longer imaginary line(likewise, not shown) extending longitudinally between source/drainzones 60 and 62 along the upper semiconductor surface. One end of thenon-zero portion of the longer line terminates at halo pocket 244. Theother end of the non-zero portion of the longer line terminates close tosource/drain zone 60.

As indicated by the solid-line portion of curve segment 64* in FIG. 42a,the net surface dopant concentration of channel zone 64 in IGFET 240increases in moving away from the location of the local surface minimumlongitudinally toward source/drain zone 62 and longitudinally reaches alocal surface maximum close to source/drain extension 62E. Thelongitudinal local surface maximum in the net surface dopantconcentration of channel zone 64 of IGFET 42 is, of course, caused byhalo pocket 244. In moving from the location of the local surfaceminimum longitudinally toward source/drain zone 60, the net surfacedopant concentration in channel zone 64 of IGFET 240 drops to zero uponreaching source/drain zone 60. This is indicated by the dashed-line partof curve segment 64* in FIG. 42a. Hence, the surface dopant profile inchannel zone 64 of IGFET 240 is longitudinally shaped like a long halfsaddle.

Along the upper semiconductor surface, the net dopant concentration ofchannel zone 84 in short-channel IGFET 242 has a longitudinal shape thatbasically constitutes the limit of the net surface dopant concentrationof channel zone 64 in long-channel IGFET 240 as the channel length ismade quite small. In this regard, FIG. 42b presents an example of hownet dopant concentration Nu in short-channel IGFET 42 varies along theupper semiconductor surface in the longitudinal direction. Aside fromdifferences caused by the presence of asymmetrical halo pocket 246rather than symmetrical extended halo pocket 104, the surface dopantprofile in FIG. 42b for IGFET 242 is substantially the same as thesurface dopant profile in FIG. 10b for IGFET 42. Horizontal distance xin FIG. 42b is measured the same as in FIG. 10b. Curve segment 246* inFIG. 42b represents the net dopant concentration in halo pocket 246along the upper semiconductor surface.

The solid-line portion of curve segment 84* in FIG. 42b represents thenet surface dopant concentration in the right half of channel zone 84from the channel center to source/drain zone 82. The dashed-line portionof curve segment 84* in FIG. 42b represents the net surface dopantconcentration in the left half of channel zone 84 from the channelcenter to source/drain zone 80. Curve segment 84* is also labeled 246*since halo pocket 246 extends fully across channel zone 84.

As indicated by the solid-line and dashed-line portions of curve segment84* in FIG. 42b, the net surface dopant concentration of channel zone 84in IGFET 242 longitudinally reaches a local surface maximum along theupper semiconductor surface at a location near source/drain zone 82 andlongitudinally decreases in going from the location of the longitudinallocal surface minimum toward source/drain zone 80. The longitudinallocal surface maximum in the net surface dopant concentration of channelzone 84 in IGFET 242 is, of course, caused by halo pocket 246. Unlikelong-channel IGFET 240, the net dopant concentration of channel zone 84in short-channel IGFET 242 normally does not longitudinally reach alocal surface minimum along the upper semiconductor surface betweensource/drain zones 80 and 82.

The p-type halo implant for asymmetrical IGFET 240 or 242 is performedso that the halo ions pass directly by only one of the oppositetransverse sides of the IGFET's then-existent gate electrode structure(68P/208 or 88P/208) and into the underlying monosilicon to form halopocket 244 or 246. In the example of FIG. 41a or 41 b, the halo ionspass directly by only the right-hand transverse side of thethen-existent gate electrode structure of IGFET 240 or 242. A suitabletechnique is employed to present the halo ions from passing directly bythe other transverse side of the IGFETs then-existent gate electrodestructure.

The mask-control technique disclosed in Wang et al, U.S. patentapplication Ser. No. 09/540,734, cited above, can be utilized to formhalo pocket 244 or 246 when the p-type halo implantation is performed byangled implantation. In this case, the shape of opening 230 inphotoresist mask 220 is controlled in the manner described above inconnection with Eq. 21 and the immediately following material so thations of the p-type halo dopant pass directly by only one of the oppositetransverse sides of the then-existent gate electrode structure of IGFET240 or 242 without significantly impairing the ability of the ions ofthe n-type source/drain extension dopant to pass directly by bothtransverse sides of the then-existent gate electrode structure of IGFET240 or 242 and into the underlying monosilicon to form a source/drainextensions 60E and 62E or 80E and 82E.

The preceding mask-control technique is likewise employed when thesemiconductor structure is to contain one or more asymmetrical long orshort p-channel IGFETs configured in a similar, but complementary,manner to n-channel IGFET 240 or 242. By fabricating the asymmetricalIGFETs as described here, one or more of symmetrical IGFETs 40, 42, 120,and 122 can be readily incorporated into a semiconductor structurecontaining one or more of asymmetrical n-channel IGFETs 240 and 242 andtheir asymmetrical p-channel counterparts.

Asymmetrical IGFET 242 achieves short-channel threshold voltage roll-offcompensation in largely the same way as symmetrical IGFET 42. When IGFET242 is provided in the same semiconductor structure as IGFET 42, thedosage of the p-type halo ions impinging on the upper semiconductorsurface area allocated for IGFET 242 is approximately the same as thedosage of the p-type halo ions impinging on the upper semiconductor areaallocated for IGFET 42. Since the p-type halo ions pass directly by onlyone of the transverse sides of the then-existent gate electrodestructure of IGFET 242, the total amount of p-type halo dopant inchannel zone 84 of IGFET 242 per unit drain width is typicallyapproximately half the total amount of p-type halo dopant in channelzone 84 of IGFET 42 per unit drain width. At the same short-channelvalue of channel length L, the total increase in average net channeldopant concentration N_(C) for IGFET 242 is thus approximately half thetotal increase in concentration N_(C) for IGFET 42. This may lead to aslight difference in the short-channel threshold voltage roll-offcompensation between IGFETs 242 and 42. Asymmetrical IGFET 242 willnormally have a slightly lower threshold voltage than symmetrical IGFET42.

When asymmetrical IGFET 242 is formed in the same semiconductorstructure as symmetrical IGFET 42, a blocking mask can be placed overthe upper semiconductor surface area allocated for IGFET 42 during partof the p-type halo implantation. By appropriately choosing the amount ofp-type halo dopant blocked from entering channel zone 84 of IGFET 42,IGFETs 242 and 42 can achieve roughly the same amount of short-channelthreshold voltage roll-off compensation at the same channel length.IGFETs 242 and 42 should then have roughly the same threshold voltage.

Asymmetrical IGFET 240 or 242 is typically utilized in the situationwhere one of source/drain zones 60 and 62 or 80 and 82 permanentlyserves as the source, while the other of source/drain zones 60 and 62 or80 and 82 permanently serves as the drain. In this case, halo pocket 244or 246 is normally placed along the source. The same applies toasymmetrical long and short p-channel IGFETs configured in a similar,but complementary, manner to n-channel IGFETs 240 and 242.

IGFETs 240 and 242 can be modified to include the p-type isolationdopant of IGFETs 110 and 120. The same likewise applies to theasymmetrical p-channel counterparts of n-channel IGFETs 240 and 242.Short-channel threshold voltage roll-off is alleviated in suchisolation-dopant variations of n-channel IGFETs 240 and 242 and theirasymmetrical p-channel counterparts in substantially the same way as inasymmetrical IGFETs 240 and 242 and thus in largely the same way as insymmetrical IGFETs 40 and 42.

Variations

While the invention has been described with reference to particularembodiments, this description is solely for the purpose of illustrationand is not to be construed as limiting the scope of the inventionclaimed below. For example, in a situation where one of the source/drainzones of an IGFET serves permanently as the source while the othersource/drain zone serves permanently as the drain, a source/drainextension zone in an IGFET of the invention may only be provided as partof the drain so that the IGFET is an asymmetrical device. If the IGFETis also asymmetrical in that ions of the halo dopant of the IGFET passdirectly by only one of the transverse sides of the IGFET'sthen-existent gate electrode structure during the halo implantation, ahalo pocket portion for the IGFET is then normally present only along,or primarily along, the source.

Any of the IGFETs configured according to the invention may, subject towhat is stated above, generally be present in the same semiconductorstructure as any other IGFET configured according to the inventionregardless of (a) whether the two IGFETs are of the same or oppositepolarity type, (b) the channel lengths of the IGFETs, and (c) thesymmetrical/asymmetrical nature of the IGFETs. Similarly, a capabilityfor manufacturing any two IGFETs configured according to the inventionmay be incorporated into an implementation of the present fabricationprocess regardless of whether the two IGFETs are actually present in anysingle semiconductor structure manufactured according to the invention.

The present semiconductor structure may include like-polarity IGFETswhose channel lengths extend approximately perpendicular to each other.In that case, the angled halo ion implantation for the IGFETs mayinclude two additional parts performed at suitable azimuthalorientations, normally opposite to each other, i.e., 180° apart, andtypically respectively perpendicular, i.e., at 90°, to the other twoparts of the angled implantation. When the azimuthal angles for the fourparts of the halo implantation respectively differ from 0°, 90°, 180°and 270° relative to a reference line extending in the longitudinaldirection of one of the IGFETs, the angled ion implantation at eachconsecutive pair of azimuthal angles typically at least partiallyovercomes transverse halo asymmetry that would otherwise result fromutilizing only two opposite ones of the azimuthal angles. In such acase, preferred values for the azimuthal angles may be 45°, 135°, 225°,and 315°.

The fabrication process of the invention can be combined with one ormore other process modules to produce semiconductor structures havinggate dielectric layers of two or more significantly differentthicknesses. Taking note that the fabrication process of FIG. 16 at anominal gate dielectric thickness of 4 nm forms a low-voltage processmodule, a high-voltage process module at a greater nominal gatedielectric thickness, e.g., 7 nm, can be combined with the fabricationprocess of FIG. 16 at the nominal gate dielectric thickness of 4 nm.Various modifications and applications may thus be made by those skilledin the art without departing from the true scope and spirit of theinvention as defined in the appended claims.

We claim:
 1. A method comprising: providing a gate electrode above agate dielectric layer that overlies a location for a channel zone in asemiconductor body having an upper surface; performing one of (a)introducing first p-type semiconductor dopant into the semiconductorbody at a first dosage and to a first average depth below the body'supper surface and (b) introducing second p-type semiconductor dopantinto the semiconductor body at a second dosage and to a second averagedepth below the body's upper surface, the first dosage being greaterthan the second dosage, the second average depth being greater than thefirst average depth; and performing the other of the introducing actssuch that the first p-type dopant defines a pair of laterally separatedp-type main source/drain portions of a pair of respective p-typesource/drain zones laterally separated by the channel zone, such thatthe second p-type dopant defines a pair of laterally separated p-typelower source/drain portions respectively of the source/drain zones, andsuch that the lower portions are more lightly doped than, respectivelyunderlie, and are respectively vertically continuous with the mainportions.
 2. A method as in claim 1 wherein the acts of introducing thefirst and second p-type dopants comprise introducing the first andsecond p-type dopants through an opening in a primary mask provided overthe semiconductor body and into the semiconductor body using the primarymask, the gate electrode, and any material along the gate electrode as aprimary dopant-blocking shield.
 3. A method as in claim 2 furtherincluding, after the providing act and before the introducing acts:introducing third p-type semiconductor dopant through an opening in afurther mask provided over the semiconductor body and into thesemiconductor body using the further mask, the gate electrode, and anymaterial along the gate electrode as a further dopant-blocking shield;and providing spacer material to the transverse sides of the gateelectrode such that the primary dopant-blocking shield includes thespacer material and such that each source/drain zone further includes asource/drain extension more lightly doped than, and laterally continuouswith, that source/drain zone's main portion, the channel zone beingterminated by the extensions along the body's upper surface.
 4. A methodas in claim 2 wherein: the act of introducing the first p-type dopantcomprises implanting ions of a species of the first p-type dopant; andthe act of introducing the second p-type dopant comprises implantingions of a species of the second p-type dopant.
 5. A method as in claim 4wherein the species of the first and second p-type dopant are different,the first and second p-type dopants being the same.
 6. A method as inclaim 5 wherein the species of the first p-type dopant is of greatermolecular weight than the species of the second p-type dopant.
 7. Amethod as in claim 5 wherein: the first and second p-type dopants areboron; the species of the first p-type dopant is a boron-containingcompound; and the species of the second p-type dopant is elementalboron.
 8. A method as in claim 7 wherein the boron-containing compoundis boron difluoride.
 9. A method as in claim 1 wherein the channel zoneis of n-type conductivity, whereby the transistor is of enhancement-modetype.
 10. A method of fabricating a principal p-channel field-effecttransistor from a semiconductor body having n-type body material, themethod comprising: introducing first n-type semiconductor dopant intothe body material; subsequently providing a gate electrode above, andvertically separated by gate dielectric material from, a portion of thebody material intended to be a channel zone for the transistor such thatthe semiconductor body has an upper surface along which there is alocation for a channel surface depletion region that extends into thechannel zone during transistor operation, such that the first n-typedopant reaches a maximum concentration below the location for thechannel surface depletion region, and such that the maximumconcentration of the first n-type dopant occurs more than 0.1 μm deepinto the body material but not more than 0.4 μm deep into the bodymaterial; and subsequently performing both of (a) introducing secondn-type semiconductor dopant into at least the intended channel-zoneportion of the body material and (b) introducing p-type semiconductordopant into the semiconductor body to form a pair of p-type source/drainzones laterally separated by the channel zone such that the secondn-type dopant has a concentration which, in the channel zone along thebody's upper surface, longitudinally reaches a local surface minimum ata location between the source/drain zones, the act of introducing p-typedopant comprising: performing one of (i) introducing first p-typesemiconductor dopant into the semiconductor body at a first dosage andto a first average depth below the body's upper surface and (ii)introducing second p-type semiconductor dopant into the semiconductorbody at a second dosage and to a second average depth below the body'supper surface, the first dosage being greater than the second dosage,the second average depth being greater than the first average depth; andperforming the other of the acts of introducing the first and secondp-type dopants such that each source/drain zone comprises a mainsource/drain portion and a more lightly doped lower source/drain portionunderlying, and vertically continuous with, the main portion.
 11. Amethod as in claim 10 wherein the act of introducing the first n-typedopant comprises implanting ions of a species of the first n-type dopantat a selected implantation energy, the transistor having a thresholdvoltage that is controlled by adjusting the implantation energy.
 12. Amethod as in claim 11 wherein the threshold voltage decreases inmagnitude as the implantation energy increases at a given dosage of thefirst n-type dopant.
 13. A method as in claim 11 wherein the thresholdvoltage varies with the implantation energy largely independent ofchannel length of the transistor.
 14. A method as in claim 11 wherein:the implanting act comprises implanting ions of the species of the firstn-type dopant through an opening in a first mask provided over thesemiconductor body, the opening in the first mask overlying the intendedchannel-zone portion of the body material, the first mask covering aportion of the body material intended to be a channel zone of anadditional p-channel field-effect transistor; and the method furtherincludes, prior to the gate-electrode providing act, implanting ions ofthe species of the first n-type dopant through an opening in a secondmask provided over the semiconductor body and into the body material atan additional implantation energy different from the selectedimplantation energy, the opening in the second mask overlying theportion of the body material intended to be the channel zone of theadditional transistor, the second mask covering the portion of the bodymaterial intended to be the channel zone of the principal transistor,the first n-type dopant reaching a maximum concentration below alocation for a channel surface depletion region that extends along theupper semiconductor surface into the channel zone of the additionaltransistor during transistor operation such that the transistors havedifferent threshold voltages due, at least, to the differentimplantation energies.
 15. A method as in claim 10 wherein the act ofintroducing the second n-type dopant comprises implanting ions of aspecies of the second n-type dopant at an average tilt angle of at least15° relative to a direction generally perpendicular to the body's uppersurface.
 16. A method as in claim 15 wherein: one part of the ions ofthe species of the second n-type dopant impinge on the semiconductorbody at an average azimuthal angle which differs from 0° by no more than60°; another part of the ions of the species of the second n-type dopantimpinge on the semiconductor body at an average azimuthal angle whichdiffers from 180° by no more than 60°; and the azimuthal angles aremeasured in a plane extending generally parallel to the body's uppersurface and from a reference line extending generally along channellength of the transistor.
 17. A method as in claim 10 wherein: the actof introducing the second n-type dopant comprises implanting ions of aspecies of the second n-type dopant through an opening in a maskprovided over the semiconductor body; and the acts of introducing thefirst and second p-type dopants comprise implanting species of the firstand second p-type dopants through the opening in the mask to at leastpartially form the source/drain zones.
 18. A method as in claim 10wherein the act of introducing p-type dopant comprises: introducingthird p-type semiconductor dopant through an opening in a first mask,through the body's upper surface, and into a pair of laterally separatedprimary segments of the semiconductor body using the first mask, thegate electrode, and any material along the gate electrode as adopant-blocking shield; providing spacer material to the transversesides of the gate electrode; and introducing the first and second p-typesemiconductor dopants through an opening in a second mask, through thebody's upper surface, and into a pair of laterally separated furthersegments of the semiconductor body using the second mask, the gateelectrode, and the spacer material as a dopant-blocking shield.
 19. Amethod as in claim 10 wherein: the first p-type dopant defines the mainsource/drain portions; and the second p-type dopant defines the lowersource/drain portions.
 20. A method as in claim 19 wherein: the act ofintroducing the first p-type dopant comprises implanting ions of aspecies of the first p-type dopant; and the act of introducing thesecond p-type dopant comprises implanting ions of a species of thesecond p-type dopant.
 21. A method as in claim 20 wherein the first andsecond p-type dopants are different.
 22. A method as in claim 20 whereinthe species of the first and second p-type dopants are different, thefirst and second p-type dopants being the same.
 23. A method as in claim22 wherein: the first and second p-type dopants are boron; the speciesof the first p-type dopant is boron difluoride; and the species of thesecond p-type dopant is elemental boron.
 24. A method as in claim 1wherein: the act of introducing the first p-type dopant comprisesimplanting ions of boron difluoride at an implantation of energy of20-30 KeV; and the act of introducing the second p-type dopant comprisesimplanting ions of boron at an implantation energy of 20-35 KeV.
 25. Amethod as in claim 1 wherein: the act of introducing the first p-typedopant comprises implanting ions of a species of the first p-type dopantat a dosage of 2.5×10¹⁵-1×10¹⁶ ions/cm²; and the act of introducing thesecond p-type dopant comprises implanting ions of a species of thesecond p-type dopant at a dosage of 3×10¹³-6×10¹³ ions/cm².
 26. A methodas in claim 1 wherein: the act of introducing the first p-type dopantcomprises implanting ions of boron difluoride at a dosage of2.5×10¹⁵-1×10¹⁶ ions/cm² and an implantation energy of 20-30 KeV; andthe act of introducing the second p-type dopant comprises implantingions of boron at a dosage of 3×10¹³-6×10¹³ ions/cm² and an implantationenergy of 20-35 KeV.